Make order-related service objects configurable

core/app/models/spree/order.rb

@@ -269,15 +269,15 @@ def all_inventory_units_returned?
     end
 
     def contents
-      @contents ||= Spree::OrderContents.new(self)
+      @contents ||= Spree::Config.order_contents_class.new(self)
     end
 
     def shipping
-      @shipping ||= Spree::OrderShipping.new(self)
+      @shipping ||= Spree::Config.order_shipping_class.new(self)
     end
 
     def cancellations
-      @cancellations ||= Spree::OrderCancellations.new(self)
+      @cancellations ||= Spree::Config.order_cancellations_class.new(self)
     end
 
     # Associates the specified user with the order.

core/lib/spree/app_configuration.rb

@@ -372,6 +372,28 @@ def default_pricing_options
     #   Spree::Wallet::DefaultPaymentBuilder.
     class_name_attribute :default_payment_builder_class, default: 'Spree::Wallet::DefaultPaymentBuilder'
 
+    # Allows providing your own class for managing the contents of an order.
+    #
+    # @!attribute [rw] order_contents_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::OrderContents.
+    class_name_attribute :order_contents_class, default: 'Spree::OrderContents'
+
+    # Allows providing your own class for shipping an order.
+    #
+    # @!attribute [rw] order_shipping_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::OrderShipping.
+    class_name_attribute :order_shipping_class, default: 'Spree::OrderShipping'
+
+    # Allows providing your own class for managing the inventory units of a
+    # completed order.
+    #
+    # @!attribute [rw] order_cancellations_class
+    # @return [Class] a class with the same public interfaces as
+    #   Spree::OrderCancellations.
+    class_name_attribute :order_cancellations_class, default: 'Spree::OrderCancellations'
+
     # Allows providing your own class for canceling payments.
     #
     # @!attribute [rw] payment_canceller