diff --git a/core/app/models/spree/order.rb b/core/app/models/spree/order.rb
index 29bb53f0b73..9a62af0973f 100644
--- a/core/app/models/spree/order.rb
+++ b/core/app/models/spree/order.rb
@@ -502,7 +502,8 @@ def create_proposed_shipments
     end
 
     def create_shipments_for_line_item(line_item)
-      units = Spree::Stock::InventoryUnitBuilder.new(self).missing_units_for_line_item(line_item)
+      units = Spree::Config.stock.inventory_unit_builder_class.new(self).missing_units_for_line_item(line_item)
+
       Spree::Config.stock.coordinator_class.new(self, units).shipments.each do |shipment|
         shipments << shipment
       end
@@ -789,9 +790,12 @@ def require_email
 
     def ensure_inventory_units
       if has_checkout_step?("delivery")
-        inventory_validator = Spree::Stock::InventoryValidator.new
+        inventory_validator = Spree::Config.stock.inventory_validator_class.new
+
+        errors = line_items.map { |line_item|
+          inventory_validator.validate(line_item)
+        }.compact
 
-        errors = line_items.map { |line_item| inventory_validator.validate(line_item) }.compact
         raise InsufficientStock if errors.any?
       end
     end
@@ -809,8 +813,15 @@ def ensure_promotions_eligible
     end
 
     def validate_line_item_availability
-      availability_validator = Spree::Stock::AvailabilityValidator.new
-      raise InsufficientStock unless line_items.all? { |line_item| availability_validator.validate(line_item) }
+      availability_validator = Spree::Config.stock.availability_validator_class.new
+
+      # NOTE: This code assumes that the availability validator will return
+      # true for success and false for failure. This is not normally the
+      # behaviour of validators, as the framework only cares about the
+      # population of the errors, not the return value of the validate method.
+      raise InsufficientStock unless line_items.all? { |line_item|
+        availability_validator.validate(line_item)
+      }
     end
 
     def ensure_line_items_present
diff --git a/core/app/models/spree/stock/simple_coordinator.rb b/core/app/models/spree/stock/simple_coordinator.rb
index de523ef804d..60913cf372d 100644
--- a/core/app/models/spree/stock/simple_coordinator.rb
+++ b/core/app/models/spree/stock/simple_coordinator.rb
@@ -24,7 +24,8 @@ class SimpleCoordinator
 
       def initialize(order, inventory_units = nil)
         @order = order
-        @inventory_units = inventory_units || InventoryUnitBuilder.new(order).units
+        @inventory_units =
+          inventory_units || Spree::Config.stock.inventory_unit_builder_class.new(order).units
         @splitters = Spree::Config.environment.stock_splitters
 
         filtered_stock_locations = Spree::Config.stock.location_filter_class.new(Spree::StockLocation.all, @order).filter
diff --git a/core/lib/spree/core/stock_configuration.rb b/core/lib/spree/core/stock_configuration.rb
index b4da441742a..e3a3c9d07c2 100644
--- a/core/lib/spree/core/stock_configuration.rb
+++ b/core/lib/spree/core/stock_configuration.rb
@@ -8,6 +8,9 @@ class StockConfiguration
       attr_writer :location_filter_class
       attr_writer :location_sorter_class
       attr_writer :allocator_class
+      attr_writer :inventory_unit_builder_class
+      attr_writer :availability_validator_class
+      attr_writer :inventory_validator_class
 
       def coordinator_class
         @coordinator_class ||= '::Spree::Stock::SimpleCoordinator'
@@ -33,6 +36,21 @@ def allocator_class
         @allocator_class ||= '::Spree::Stock::Allocator::OnHandFirst'
         @allocator_class.constantize
       end
+
+      def inventory_unit_builder_class
+        @inventory_unit_builder_class ||= '::Spree::Stock::InventoryUnitBuilder'
+        @inventory_unit_builder_class.constantize
+      end
+
+      def availability_validator_class
+        @availability_validator_class ||= '::Spree::Stock::AvailabilityValidator'
+        @availability_validator_class.constantize
+      end
+ 
+      def inventory_validator_class
+        @inventory_validator_class ||= '::Spree::Stock::InventoryValidator'
+        @inventory_validator_class.constantize
+      end
     end
   end
 end
diff --git a/core/spec/lib/spree/core/stock_configuration_spec.rb b/core/spec/lib/spree/core/stock_configuration_spec.rb
index 0c03644880e..8b082840c1b 100644
--- a/core/spec/lib/spree/core/stock_configuration_spec.rb
+++ b/core/spec/lib/spree/core/stock_configuration_spec.rb
@@ -3,93 +3,153 @@
 require 'rails_helper'
 
 RSpec.describe Spree::Core::StockConfiguration do
+  let(:stock_configuration) { described_class.new }
+
   describe '#coordinator_class' do
-    let(:stock_configuration) { described_class.new }
     subject { stock_configuration.coordinator_class }
 
-    it "returns Spree::Stock::Coordinator" do
-      is_expected.to be ::Spree::Stock::SimpleCoordinator
+    it "returns Spree::Stock::Coordinator by default" do
+      expect(subject).to be ::Spree::Stock::SimpleCoordinator
     end
 
-    context "with another constant name assiged" do
+    it "can be reassigned" do
       MyCoordinator = Class.new
-      before { stock_configuration.coordinator_class = MyCoordinator.to_s }
+      stock_configuration.coordinator_class = MyCoordinator.to_s
+
+      expect(subject).to be MyCoordinator
 
-      it "returns the constant" do
-        is_expected.to be MyCoordinator
-      end
+    ensure
+      Object.send(:remove_const, :MyCoordinator)
     end
   end
 
   describe '#estimator_class' do
-    let(:stock_configuration) { described_class.new }
     subject { stock_configuration.estimator_class }
 
     it "returns Spree::Stock::Estimator" do
-      is_expected.to be ::Spree::Stock::Estimator
+      expect(subject).to be ::Spree::Stock::Estimator
     end
 
-    context "with another constant name assiged" do
+    it "can be reassigned" do
       MyEstimator = Class.new
-      before { stock_configuration.estimator_class = MyEstimator.to_s }
+      stock_configuration.estimator_class = MyEstimator.to_s
+
+      expect(subject).to be MyEstimator
 
-      it "returns the constant" do
-        is_expected.to be MyEstimator
-      end
+    ensure
+      Object.send(:remove_const, :MyEstimator)
     end
   end
 
   describe '#location_filter_class' do
-    let(:stock_configuration) { described_class.new }
     subject { stock_configuration.location_filter_class }
 
-    it "returns Spree::Stock::LocationFilter::Active" do
-      is_expected.to be ::Spree::Stock::LocationFilter::Active
+    it "returns Spree::Stock::LocationFilter::Active by default" do
+      expect(subject).to be ::Spree::Stock::LocationFilter::Active
     end
 
-    context "with another constant name assiged" do
+    it "can be reassigned" do
       MyFilter = Class.new
-      before { stock_configuration.location_filter_class = MyFilter.to_s }
+      stock_configuration.location_filter_class = MyFilter.to_s
+
+      expect(subject).to be MyFilter
 
-      it "returns the constant" do
-        is_expected.to be MyFilter
-      end
+    ensure
+      Object.send(:remove_const, :MyFilter)
     end
   end
 
   describe '#location_sorter_class' do
-    let(:stock_configuration) { described_class.new }
     subject { stock_configuration.location_sorter_class }
 
-    it "returns Spree::Stock::LocationSorter::Unsorted" do
-      is_expected.to be ::Spree::Stock::LocationSorter::Unsorted
+    it "returns Spree::Stock::LocationSorter::Unsorted by default" do
+      expect(subject).to be ::Spree::Stock::LocationSorter::Unsorted
     end
 
-    context "with another constant name assiged" do
+    it "can be reassigned" do
       MySorter = Class.new
-      before { stock_configuration.location_sorter_class = MySorter.to_s }
+      stock_configuration.location_sorter_class = MySorter.to_s
 
-      it "returns the constant" do
-        is_expected.to be MySorter
-      end
+      expect(subject).to be MySorter
+
+    ensure
+      Object.send(:remove_const, :MySorter)
     end
   end
 
   describe '#allocator_class' do
-    let(:stock_configuration) { described_class.new }
     subject { stock_configuration.allocator_class }
 
-    it "returns Spree::Stock::Allocator::OnHandFirst" do
-      is_expected.to be ::Spree::Stock::Allocator::OnHandFirst
+    it "returns Spree::Stock::Allocator::OnHandFirst by default" do
+      expect(subject).to be ::Spree::Stock::Allocator::OnHandFirst
     end
 
-    context "with another constant name assiged" do
+    it "can be reassigned" do
       MyAllocator = Class.new
-      before { stock_configuration.allocator_class = MyAllocator.to_s }
+      stock_configuration.allocator_class = MyAllocator.to_s
+
+      expect(subject).to be MyAllocator
+
+    ensure
+      Object.send(:remove_const, :MyAllocator)
+    end
+  end
+
+  describe '#inventory_unit_builder_class' do
+    subject { stock_configuration.inventory_unit_builder_class }
+
+    it "returns Spree::Stock::InventoryUnitBuilder by default" do
+      expect(subject).to be ::Spree::Stock::InventoryUnitBuilder
+    end
+
+    it "can be reassigned" do
+      MyInventoryUnitBuilder = Class.new
+      stock_configuration.inventory_unit_builder_class = MyInventoryUnitBuilder.to_s
+
+      expect(subject).to be MyInventoryUnitBuilder
+
+    ensure
+      Object.send(:remove_const, :MyInventoryUnitBuilder)
+    end
+  end
+
+  describe '#availability_validator_class' do
+    subject { stock_configuration.availability_validator_class }
+
+    let(:stock_configuration) { described_class.new }
+
+    it "returns Spree::Stock::AvailabilityValidator" do
+      is_expected.to be ::Spree::Stock::AvailabilityValidator
+    end
+
+    it "can be reassigned" do
+      MyAvailabilityValidator = Class.new
+      stock_configuration.availability_validator_class = MyAvailabilityValidator.to_s
+
+      expect(subject).to be MyAvailabilityValidator
+
+    ensure
+      Object.send(:remove_const, :MyAvailabilityValidator)
+    end
+  end
+
+  describe '#inventory_validator_class' do
+    subject { stock_configuration.inventory_validator_class }
+
+    let(:stock_configuration) { described_class.new }
+
+    it "returns Spree::Stock::InventoryValidator" do
+      is_expected.to be ::Spree::Stock::InventoryValidator
+    end
+
+    it "can be reassigned" do
+      MyInventoryValidator = Class.new
+      stock_configuration.inventory_validator_class = MyInventoryValidator.to_s
+
+      expect(subject).to be MyInventoryValidator
 
-      it "returns the constant" do
-        is_expected.to be MyAllocator
-      end
+    ensure
+      Object.send(:remove_const, :MyInventoryValidator)
     end
   end
 end
diff --git a/core/spec/models/spree/order_spec.rb b/core/spec/models/spree/order_spec.rb
index 6a068a2eceb..ac00e2f4d03 100644
--- a/core/spec/models/spree/order_spec.rb
+++ b/core/spec/models/spree/order_spec.rb
@@ -1643,11 +1643,47 @@ def generate
   describe '#create_shipments_for_line_item' do
     subject { order.create_shipments_for_line_item(line_item) }
 
-    let(:order) { create :order_with_line_items }
+    let(:order) { create :order, shipments: [] }
     let(:line_item) { build(:line_item, order: order) }
 
     it 'creates at least one new shipment for the order' do
-      expect { subject }.to change { order.shipments.count }.by 1
+      expect { subject }.to change { order.shipments.count }.from(0).to(1)
+    end
+
+    context "with a custom inventory unit builder" do
+      before do
+        # Because the defined method runs in the context of the instance of our
+        # test inventory unit builder, not the RSpec example context, we need
+        # to make this value available as a local variable. We're using
+        # Class.new and define_method to avoid creating scope gates that would
+        # take this local variable out of scope.
+        inventory_unit = arbitrary_inventory_unit
+        TestInventoryUnitBuilder = Class.new do
+          def initialize(order)
+          end
+
+          define_method(:missing_units_for_line_item) { |line_item|
+            [inventory_unit]
+          }
+        end
+
+        test_stock_config = Spree::Core::StockConfiguration.new
+        test_stock_config.inventory_unit_builder_class = TestInventoryUnitBuilder.to_s
+        stub_spree_preferences(stock: test_stock_config)
+      end
+
+      after do
+        Object.send(:remove_const, :TestInventoryUnitBuilder)
+      end
+
+      let(:arbitrary_inventory_unit) { build :inventory_unit, order: order, line_item: line_item, variant: line_item.variant }
+
+      it "relies on the custom builder" do
+        expect { subject }.to change { order.shipments.count }.from(0).to(1)
+
+        expect(order.shipments.order(:created_at).first.inventory_units)
+          .to contain_exactly arbitrary_inventory_unit
+      end
     end
   end
 
@@ -1665,4 +1701,85 @@ def generate
       expect(subject).to eq 40
     end
   end
+
+  describe "#ensure_inventory_units" do
+    subject { order.send(:ensure_inventory_units) }
+
+    before do
+      class TestValidator
+        def validate(line_item)
+          if line_item.quantity != 1
+            line_item.errors.add(:quantity, ":(")
+          end
+        end
+      end
+
+      test_stock_config = Spree::Core::StockConfiguration.new
+      test_stock_config.inventory_validator_class = TestValidator.to_s
+      stub_spree_preferences(stock: test_stock_config)
+    end
+
+    let(:order) { create :order_with_line_items, line_items_count: 2 }
+
+    it "uses the configured validator" do
+      expect_any_instance_of(TestValidator).to receive(:validate).twice.and_call_original
+
+      subject
+    end
+
+    context "when the line items are valid" do
+      it "doesn't raise an exception" do
+        expect { subject }.not_to raise_error
+      end
+    end
+
+    context "when the line items are not valid" do
+      before do
+        order.line_items.last.quantity = 2
+      end
+
+      it "raises an exception" do
+        expect { subject }.to raise_error(Spree::Order::InsufficientStock)
+      end
+    end
+  end
+
+  describe "#validate_line_item_availability" do
+    subject { order.send(:validate_line_item_availability) }
+
+    before do
+      class TestValidator
+        def validate(line_item)
+          if line_item.variant.sku == "UNAVAILABLE"
+            line_item.errors.add(:quantity, ":(")
+            false
+          else
+            true
+          end
+        end
+      end
+
+      test_stock_config = Spree::Core::StockConfiguration.new
+      test_stock_config.availability_validator_class = TestValidator.to_s
+      stub_spree_preferences(stock: test_stock_config)
+    end
+
+    let(:order) { create :order_with_line_items, line_items_count: 2 }
+
+    context "when the line items are valid" do
+      it "doesn't raise an exception" do
+        expect { subject }.not_to raise_error
+      end
+    end
+
+    context "when the line items are not valid" do
+      before do
+        order.line_items.last.variant.sku = "UNAVAILABLE"
+      end
+
+      it "raises an exception" do
+        expect { subject }.to raise_error(Spree::Order::InsufficientStock)
+      end
+    end
+  end
 end
diff --git a/core/spec/models/spree/stock/simple_coordinator_spec.rb b/core/spec/models/spree/stock/simple_coordinator_spec.rb
index 34554740302..02a245b1288 100644
--- a/core/spec/models/spree/stock/simple_coordinator_spec.rb
+++ b/core/spec/models/spree/stock/simple_coordinator_spec.rb
@@ -38,6 +38,14 @@ module Stock
           subject.shipments.count == StockLocation.count
         end
 
+        it 'uses the pluggable inventory unit builder class' do
+          expect(Spree::Config.stock)
+            .to receive(:inventory_unit_builder_class)
+            .and_call_original
+
+          subject.shipments
+        end
+
         context "missing stock items in active stock location" do
           let!(:another_location) { create(:stock_location, propagate_all_variants: false) }
 
diff --git a/frontend/app/controllers/spree/checkout_controller.rb b/frontend/app/controllers/spree/checkout_controller.rb
index afd71eecdf7..7dc1d71629f 100644
--- a/frontend/app/controllers/spree/checkout_controller.rb
+++ b/frontend/app/controllers/spree/checkout_controller.rb
@@ -230,7 +230,7 @@ def insufficient_stock_error
         flash[:error] = I18n.t('spree.insufficient_stock_for_order')
         redirect_to cart_path
       else
-        availability_validator = Spree::Stock::AvailabilityValidator.new
+        availability_validator = Spree::Config.stock.availability_validator_class.new
         unavailable_items = @order.line_items.reject { |line_item| availability_validator.validate(line_item) }
         if unavailable_items.any?
           item_names = unavailable_items.map(&:name).to_sentence