diff --git a/api/spec/requests/spree/api/orders_spec.rb b/api/spec/requests/spree/api/orders_spec.rb
index 4ca347a2ab..4d3a541022 100644
--- a/api/spec/requests/spree/api/orders_spec.rb
+++ b/api/spec/requests/spree/api/orders_spec.rb
@@ -77,20 +77,6 @@ module Spree::Api
             end
           end
         end
-
-        context "with existing promotion" do
-          let(:discount) { 2 }
-          before do
-            create(:promotion, :with_line_item_adjustment, apply_automatically: true, adjustment_rate: discount )
-          end
-
-          it "activates the promotion" do
-            post spree.api_orders_path, params: { order: { line_items: { "0" => { variant_id: variant.to_param, quantity: 1 } } } }
-            order = Spree::Order.last
-            line_item = order.line_items.first
-            expect(order.total).to eq(line_item.price - discount)
-          end
-        end
       end
 
       context "when the current user can administrate the order" do
diff --git a/backend/spec/controllers/spree/admin/orders_controller_spec.rb b/backend/spec/controllers/spree/admin/orders_controller_spec.rb
index 491c2b9d52..a9b4028198 100644
--- a/backend/spec/controllers/spree/admin/orders_controller_spec.rb
+++ b/backend/spec/controllers/spree/admin/orders_controller_spec.rb
@@ -27,7 +27,7 @@
 
     before do
       allow(Spree::Order).to receive_message_chain(:includes, find_by!: order)
-      allow(order).to receive_messages(contents: Spree::OrderContents.new(order))
+      allow(order).to receive_messages(contents: Spree::Config.order_contents_class.new(order))
     end
 
     context "#approve" do
diff --git a/core/lib/spree/app_configuration.rb b/core/lib/spree/app_configuration.rb
index b10ddde992..37d48e16e2 100644
--- a/core/lib/spree/app_configuration.rb
+++ b/core/lib/spree/app_configuration.rb
@@ -394,7 +394,7 @@ def default_pricing_options
     # @!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'
+    class_name_attribute :order_contents_class, default: 'Spree::SimpleOrderContents'
 
     # Allows providing your own class for shipping an order.
     #
diff --git a/core/spec/lib/spree/app_configuration_spec.rb b/core/spec/lib/spree/app_configuration_spec.rb
index 7ffc0e84d6..6770b9c556 100644
--- a/core/spec/lib/spree/app_configuration_spec.rb
+++ b/core/spec/lib/spree/app_configuration_spec.rb
@@ -36,6 +36,10 @@
     expect(prefs.variant_search_class).to eq Spree::Core::Search::Variant
   end
 
+  it "uses simple order contents class by default" do
+    expect(prefs.order_contents_class).to eq Spree::SimpleOrderContents
+  end
+
   it "uses variant price selector class by default" do
     expect(prefs.variant_price_selector_class).to eq Spree::Variant::PriceSelector
   end
diff --git a/core/spec/models/spree/promotion_spec.rb b/core/spec/models/spree/promotion_spec.rb
index 449e06730b..c4b696192f 100644
--- a/core/spec/models/spree/promotion_spec.rb
+++ b/core/spec/models/spree/promotion_spec.rb
@@ -915,31 +915,6 @@
     end
   end
 
-  describe "adding items to the cart" do
-    let(:order) { create :order }
-    let(:line_item) { create :line_item, order: order }
-    let(:promo) { create :promotion_with_item_adjustment, adjustment_rate: 5, code: 'promo' }
-    let(:promotion_code) { promo.codes.first }
-    let(:variant) { create :variant }
-
-    it "updates the promotions for new line items" do
-      expect(line_item.adjustments).to be_empty
-      expect(order.adjustment_total).to eq 0
-
-      promo.activate order: order, promotion_code: promotion_code
-      order.recalculate
-
-      expect(line_item.adjustments.size).to eq(1)
-      expect(order.adjustment_total).to eq(-5)
-
-      other_line_item = order.contents.add(variant, 1, currency: order.currency)
-
-      expect(other_line_item).not_to eq line_item
-      expect(other_line_item.adjustments.size).to eq(1)
-      expect(order.adjustment_total).to eq(-10)
-    end
-  end
-
   describe "promotion deletion" do
     subject { promotion.destroy! }
 
diff --git a/core/app/models/spree/order_contents.rb b/legacy_promotions/app/models/spree/order_contents.rb
similarity index 100%
rename from core/app/models/spree/order_contents.rb
rename to legacy_promotions/app/models/spree/order_contents.rb
diff --git a/core/spec/models/spree/order_contents_spec.rb b/legacy_promotions/spec/models/spree/order_contents_spec.rb
similarity index 100%
rename from core/spec/models/spree/order_contents_spec.rb
rename to legacy_promotions/spec/models/spree/order_contents_spec.rb
diff --git a/legacy_promotions/spec/models/spree/promotion_integration_spec.rb b/legacy_promotions/spec/models/spree/promotion_integration_spec.rb
index 90ad8f194d..6ed19aba4c 100644
--- a/legacy_promotions/spec/models/spree/promotion_integration_spec.rb
+++ b/legacy_promotions/spec/models/spree/promotion_integration_spec.rb
@@ -181,4 +181,29 @@
       }.to change{ order.reload.state }.from("confirm").to("address")
     end
   end
+
+  describe "adding items to the cart" do
+    let(:order) { create :order }
+    let(:line_item) { create :line_item, order: order }
+    let(:promo) { create :promotion_with_item_adjustment, adjustment_rate: 5, code: 'promo' }
+    let(:promotion_code) { promo.codes.first }
+    let(:variant) { create :variant }
+
+    it "updates the promotions for new line items" do
+      expect(line_item.adjustments).to be_empty
+      expect(order.adjustment_total).to eq 0
+
+      promo.activate order: order, promotion_code: promotion_code
+      order.recalculate
+
+      expect(line_item.adjustments.size).to eq(1)
+      expect(order.adjustment_total).to eq(-5)
+
+      other_line_item = order.contents.add(variant, 1, currency: order.currency)
+
+      expect(other_line_item).not_to eq line_item
+      expect(other_line_item.adjustments.size).to eq(1)
+      expect(order.adjustment_total).to eq(-10)
+    end
+  end
 end
diff --git a/legacy_promotions/spec/requests/spree/api/orders_spec.rb b/legacy_promotions/spec/requests/spree/api/orders_spec.rb
new file mode 100644
index 0000000000..3fc4f700f4
--- /dev/null
+++ b/legacy_promotions/spec/requests/spree/api/orders_spec.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'API Orders', type: :request do
+  let!(:order) { create(:order) }
+  let(:variant) { create(:variant) }
+  let(:line_item) { create(:line_item) }
+  let(:address_params) { { country_id: Country.first.id, state_id: State.first.id } }
+
+  let(:current_api_user) do
+    user = Spree.user_class.new(email: "solidus@example.com")
+    user.generate_spree_api_key!
+    user
+  end
+
+  before do
+    stub_authentication!
+  end
+
+  describe "POST create" do
+    let(:target_user) { create :user }
+    let(:attributes) { { user_id: target_user.id, email: target_user.email } }
+
+    subject do
+      post spree.api_orders_path, params: { order: attributes }
+      response
+    end
+
+    context "when the current user cannot administrate the order" do
+      custom_authorization! do |_|
+        can :create, Spree::Order
+      end
+
+      context "with existing promotion" do
+        let(:discount) { 2 }
+        before do
+          create(:promotion, :with_line_item_adjustment, apply_automatically: true, adjustment_rate: discount )
+        end
+
+        it "activates the promotion" do
+          post spree.api_orders_path, params: { order: { line_items: { "0" => { variant_id: variant.to_param, quantity: 1 } } } }
+          order = Spree::Order.last
+          line_item = order.line_items.first
+          expect(order.total).to eq(line_item.price - discount)
+        end
+      end
+    end
+  end
+end