Use shipping fee adjustment as single source of truth, and persist it on the shipment :cost field when it changes.

app/models/spree/shipment.rb

@@ -161,10 +161,8 @@ module Spree
     end
 
     def update_amounts
-      return unless selected_shipping_rate
-
-      self.update_columns(
-        cost: selected_shipping_rate.cost,
+      update_columns(
+        cost: adjustment&.amount || 0.0,
         updated_at: Time.zone.now
       )
     end
@@ -278,7 +276,7 @@ module Spree
         reload # ensure adjustment is present on later saves
       end
 
-      update_amounts if adjustment
+      update_amounts if adjustment&.amount != cost
       update_adjustment_included_tax if adjustment
     end
 

spec/models/spree/shipment_spec.rb

@@ -428,17 +428,12 @@ describe Spree::Shipment do
   end
 
   describe "#update_amounts" do
-    it "updates shipping cost when selected_shipping_rate is present" do
-      allow(shipment).to receive(:selected_shipping_rate) { double(:rate, cost: 10) }
+    it "persists the shipping cost from the shipping fee adjustment" do
+      allow(shipment).to receive(:adjustment) { double(:adjustment, amount: 10) }
       expect(shipment).to receive(:update_columns).with(cost: 10, updated_at: kind_of(Time))
 
       shipment.update_amounts
     end
-
-    it "does nothing when selected_shipping_rate is not present" do
-      expect(shipment).to_not receive(:update_columns)
-      shipment.update_amounts
-    end
   end
 
   context "after_save" do