Refactor shipping_rate calculation

The easypost_shipments were being created again upon retrieving shipping_rates.
Hence modified estimator to check if an EasyPost shipment has already been created for the shipment.
If it exists, it fetches the stored shipping rates instead of creating a new EasyPost shipment.

Upon updating default shipping rate, all the other shipping rates of a shipment were being discarded.
The change ensure that the newly selected shipping rate is set while also keeping the rest.
Once the shipping rate is updated, the cost of shipment needs to be updated.
Thereby reloading shipment to reflect any updates to the selected shipping rate also ensuring that the shipment cost is recalculated based on the latest
selected shipping rate.

Author: JustShah

app/decorators/models/solidus_easypost/spree/shipment_decorator.rb

@@ -29,8 +29,41 @@ def easypost_postage_label_url
         easypost_shipment&.postage_label&.label_url
       end
 
+      def select_shipping_method(shipping_method)
+        # Selects the specified shipping method for the shipment.
+        # This method fetches available shipping rates using the estimator and
+        # marks the selected shipping rate while deselecting all others.
+        estimator = ::Spree::Config.stock.estimator_class.new
+        rates = estimator.shipping_rates(to_package, false)
+
+        # Find the rate that matches the provided shipping method
+        rate = rates.detect { |detected| detected.shipping_method_id == shipping_method.id }
+
+        # Mark the selected shipping method as true and deselect others
+        deselect_other_shipping_rates(rate.id)
+        rate.update(selected: true)
+      end
+
+      def update_amounts
+        # NOTE: Reload the shipment to reflect any updates to the selected shipping rate.
+        # Updates the shipment amounts by reloading the selected shipping rate.
+        # This ensures that the shipment cost is recalculated based on the latest
+        # selected shipping rate.
+        reload
+
+        # Call the parent method from solidus_core to ensure proper cost recalculations.
+        super
+      end
+
       private
 
+      def deselect_other_shipping_rates(selected_rate_id)
+        # Deselects all shipping rates except the one selected by the user.
+        # This ensures that only one shipping rate remains marked as selected.
+        # Update all other shipping rates, setting `selected` to false.
+        shipping_rates.where.not(id: selected_rate_id).update_all(selected: false)
+      end
+
       def buy_easypost_rate
         # Skip label purchase if tracking information already exists.
         return if tracking

lib/solidus_easypost/estimator.rb

@@ -3,10 +3,22 @@
 module SolidusEasypost
   class Estimator
     def shipping_rates(package, _frontend_only = true)
-      easypost_rates = ShipmentBuilder.from_package(package).rates.sort_by(&:rate)
-
-      shipping_rates = easypost_rates.map { |rate| build_shipping_rate(rate) }.compact
-      shipping_rates.min_by(&:cost)&.selected = true
+      # NOTE: The easypost_shipments were being created again when retrieving shipping_rates
+      # Hence this method has been updated to check if an EasyPost shipment has already been created for the shipment.
+      # If it exists, it fetches the stored shipping rates instead of creating a new EasyPost shipment.
+      shipment = package.shipment
+
+      if shipment.easypost_shipment
+        shipping_rates = shipment.shipping_rates
+      else
+        easypost_rates = ShipmentBuilder.from_package(package).rates.sort_by(&:rate)
+
+        # Build shipping rate objects from EasyPost rates.
+        shipping_rates = easypost_rates.map { |rate| build_shipping_rate(rate) }.compact
+
+        # Automatically select the shipping rate with the lowest cost.
+        shipping_rates.min_by(&:cost)&.selected = true
+      end
 
       shipping_rates
     end