Add docs that summarize shipping calculators

guides/calculators/shipping-calculators.md

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+# Shipping calculators
+
+Solidus comes with a set of default calculators that account for typical
+shipping scenarios:
+
+- [Flat percent](#flat-percent)
+- [Flat rate](#flat-rate) (per order)
+- [Flat rate per package item](#flat-rate-per-package-item)
+- [Flexi rate](#flexi-rate)
+- [Price sack](#price-sack)
+
+All of Solidus's shipping calculators inherit from the
+`Spree::ShippingCalculator` class.
+
+If the built-in calculators are not suitable for your store, you can create
+custom shipping calculators. For more information, see the [Custom shipping
+calculators][custom-shipping-calculators] article.
+
+The rest of this article summarizes the functionality of Solidus's built-in
+shipping calculators. Note that the code examples use [preferred
+methods][preferred-methods] to set preference values. You can use these methods
+to update any preference. For example:
+
+```ruby
+Spree::Calculator.find(1).update(preferred_currency: "EUR")
+```
+
+[custom-shipping-calculators]: ../shipments/custom-shipping-calculators.md
+[preferred-methods]: overview.md#preferred-methods
+
+## Flat percent
+
+The [`Spree::Calculator::Shipping::FlatPercentItemTotal`
+calculator][flat-percent] has one preference, `flat_percent`, that takes an
+integer.
+
+For each package that is shipped, this calculator takes the package's total and
+charges a percentage of it. For example:
+
+- An order has one shipment that includes three items.
+- The total of the three items is $190.
+- The calculator's `flat_percent` value is set to `10`.
+- The shipping is calculated as 10% of $190: `$190 * (10 / 100)`.
+
+## Flat rate
+
+The [`Spree::Calculator::Shipping::FlatRate` calculator][flat-rate-per-order]
+can be used to provide a flat rate discount. It has the following preferences:
+
+- `amount`: The amount per item to calculate.
+- `currency`: The currency. This value must match an available shipping method.
+  The default value is your store's default currency setting.
+
+The currency for this calculator is used to check to see if a shipping method is
+available for an order. If an order's currency does not match a shipping
+method's currency, then that shipping method is not displayed on the frontend.
+
+## Flexi rate
+
+The [`Spree::Calculator::Shipping::FlexiRate` calculator][flexi-rate] provides a
+flexible rate depending on the items in a package. (Or, a subset of specific
+items in a package.) It has the following preferences:
+
+- `first_item`: The shipping price for the first item(s).
+- `additional_item`: The shipping price for subsequent items.
+- `max_items`: The maximum number of items that the rate applies to.
+- `currency`: The currency. This value must match an available shipping method.
+  The default value is your store's default currency setting.
+
+## Flat rate per package item
+
+The [`Spree::Calculator::Shipping::PerItem` calculator][flat-rate-per-item]
+computes a value for every item on an order. This is useful for providing a
+discount for a specific product without affecting other products. It has the
+following preferences:
+
+- `amount`: The amount per item to calculate.
+- `currency`: The currency. This value must match an available shipping method.
+  The default value is your store's default currency setting.
+
+## Price sack
+
+The [`Spree::Calculator::Shipping::PriceSack` calculator][price-sack] is useful
+for when you want to provide a discount for an order which is over a certain
+price. The calculator has the following preferences:
+
+- `minimal_amount`: The minimum amount for the line items total to trigger the
+  calculator.
+- `normal_amount`: The amount to discount from the order if the line items total
+  is less than the `minimal_amount`.
+- `discount_amount`: The amount to discount from the order if the line items
+  total is equal to or greater than the `minimal_amount`.
+- `currency`: The currency. This value must match an available shipping method.
+  The default value is your store's default currency setting.
+
+[flat-percent]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/flat_percent_item_total.rb
+[flat-rate-per-order]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/flat_rate.rb
+[flat-rate-per-package-item]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/per_item.rb
+[flexi-rate]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/flexi_rate.rb
+[price-sack]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/price_sack.rb
+