diff --git a/app/controllers/api/v0/base_controller.rb b/app/controllers/api/v0/base_controller.rb
index 304fd990a43..1f89cb6c685 100644
--- a/app/controllers/api/v0/base_controller.rb
+++ b/app/controllers/api/v0/base_controller.rb
@@ -41,6 +41,10 @@ def respond_with_conflict(json_hash)
 
       private
 
+      def spree_current_user
+        @spree_current_user ||= request.env['warden'].user
+      end
+
       # Use logged in user (spree_current_user) for API authentication (current_api_user)
       def authenticate_user
         return if @current_api_user = spree_current_user
diff --git a/app/controllers/metal_decorator.rb b/app/controllers/metal_decorator.rb
deleted file mode 100644
index 9e9364fa813..00000000000
--- a/app/controllers/metal_decorator.rb
+++ /dev/null
@@ -1,8 +0,0 @@
-# frozen_string_literal: true
-
-# For the API
-ActionController::Metal.class_eval do
-  def spree_current_user
-    @spree_current_user ||= request.env['warden'].user
-  end
-end
diff --git a/app/models/calculator/price_sack.rb b/app/models/calculator/price_sack.rb
index 6ee294d6a1a..a36783afe1c 100644
--- a/app/models/calculator/price_sack.rb
+++ b/app/models/calculator/price_sack.rb
@@ -1,7 +1,5 @@
 # frozen_string_literal: false
 
-# For #to_d method on Ruby 1.8
-require 'bigdecimal/util'
 require 'spree/localized_number'
 
 module Calculator
diff --git a/app/models/concerns/calculated_adjustments.rb b/app/models/concerns/calculated_adjustments.rb
new file mode 100644
index 00000000000..af55dfa5b2f
--- /dev/null
+++ b/app/models/concerns/calculated_adjustments.rb
@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+require "active_support/concern"
+
+module CalculatedAdjustments
+  extend ActiveSupport::Concern
+
+  included do
+    has_one :calculator, as: :calculable, class_name: "Spree::Calculator", dependent: :destroy
+    accepts_nested_attributes_for :calculator
+    validates :calculator, presence: true
+  end
+
+  class_methods do
+    def calculators
+      spree_calculators.__send__(model_name_without_spree_namespace)
+    end
+
+    private
+
+    def model_name_without_spree_namespace
+      to_s.tableize.gsub('/', '_').sub('spree_', '')
+    end
+
+    def spree_calculators
+      Rails.application.config.spree.calculators
+    end
+  end
+
+  def calculator_type
+    calculator.class.to_s if calculator
+  end
+
+  def calculator_type=(calculator_type)
+    klass = calculator_type.constantize if calculator_type
+    self.calculator = klass.new if klass && !calculator.is_a?(klass)
+  end
+
+  # Creates a new adjustment for the target object
+  #   (which is any class that has_many :adjustments) and sets amount based on the
+  #   calculator as applied to the given calculable (Order, LineItems[], Shipment, etc.)
+  # By default the adjustment will not be considered mandatory
+  def create_adjustment(label, adjustable, mandatory = false, state = "closed", tax_category = nil)
+    amount = compute_amount(adjustable)
+    return if amount.zero? && !mandatory
+
+    adjustment_attributes = {
+      amount: amount,
+      originator: self,
+      order: order_object_for(adjustable),
+      label: label,
+      mandatory: mandatory,
+      state: state,
+      tax_category: tax_category
+    }
+
+    if adjustable.respond_to?(:adjustments)
+      adjustable.adjustments.create(adjustment_attributes)
+    else
+      adjustable.create_adjustment(adjustment_attributes)
+    end
+  end
+
+  # Calculate the amount to be used when creating an adjustment
+  # NOTE: May be overriden by classes where this module is included into.
+  def compute_amount(calculable)
+    calculator.compute(calculable)
+  end
+
+  private
+
+  def order_object_for(target)
+    # Temporary method for adjustments transition.
+    if target.is_a? Spree::Order
+      target
+    elsif target.respond_to?(:order)
+      target.order
+    end
+  end
+end
diff --git a/app/models/concerns/payment_method_distributors.rb b/app/models/concerns/payment_method_distributors.rb
index d9ac7ccb681..44973efb923 100644
--- a/app/models/concerns/payment_method_distributors.rb
+++ b/app/models/concerns/payment_method_distributors.rb
@@ -8,12 +8,10 @@
 module PaymentMethodDistributors
   extend ActiveSupport::Concern
 
-  def self.included(base)
-    base.class_eval do
-      has_and_belongs_to_many :distributors, join_table: 'distributors_payment_methods',
-                                             class_name: 'Enterprise',
-                                             foreign_key: 'payment_method_id',
-                                             association_foreign_key: 'distributor_id'
-    end
+  included do
+    has_and_belongs_to_many :distributors, join_table: 'distributors_payment_methods',
+                                           class_name: 'Enterprise',
+                                           foreign_key: 'payment_method_id',
+                                           association_foreign_key: 'distributor_id'
   end
 end
diff --git a/app/models/enterprise_fee.rb b/app/models/enterprise_fee.rb
index 91b262e279d..9ceed010200 100644
--- a/app/models/enterprise_fee.rb
+++ b/app/models/enterprise_fee.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class EnterpriseFee < ApplicationRecord
-  include Spree::Core::CalculatedAdjustments
+  include CalculatedAdjustments
 
   acts_as_paranoid
 
diff --git a/app/models/spree/gateway.rb b/app/models/spree/gateway.rb
index 6a0320835bd..b28350cf1b0 100644
--- a/app/models/spree/gateway.rb
+++ b/app/models/spree/gateway.rb
@@ -5,6 +5,7 @@
 
 module Spree
   class Gateway < PaymentMethod
+    acts_as_taggable
     include PaymentMethodDistributors
 
     delegate_belongs_to :provider, :authorize, :purchase, :capture, :void, :credit
diff --git a/app/models/spree/payment_method.rb b/app/models/spree/payment_method.rb
index c055842951f..26eec433a04 100644
--- a/app/models/spree/payment_method.rb
+++ b/app/models/spree/payment_method.rb
@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 
 require 'concerns/payment_method_distributors'
-require 'spree/core/calculated_adjustments'
 
 module Spree
   class PaymentMethod < ApplicationRecord
-    include Spree::Core::CalculatedAdjustments
+    include CalculatedAdjustments
     include PaymentMethodDistributors
 
     acts_as_taggable
@@ -94,10 +93,6 @@ def supports?(_source)
     end
 
     def init
-      unless _reflections.key?(:calculator)
-        self.class.include Spree::Core::CalculatedAdjustments
-      end
-
       self.calculator ||= ::Calculator::FlatRate.new(preferred_amount: 0)
     end
 
diff --git a/app/models/spree/shipping_method.rb b/app/models/spree/shipping_method.rb
index 931186f28b9..53d77d60c28 100644
--- a/app/models/spree/shipping_method.rb
+++ b/app/models/spree/shipping_method.rb
@@ -2,7 +2,7 @@
 
 module Spree
   class ShippingMethod < ApplicationRecord
-    include Spree::Core::CalculatedAdjustments
+    include CalculatedAdjustments
     DISPLAY = [:both, :front_end, :back_end].freeze
 
     acts_as_paranoid
diff --git a/app/models/spree/tax_rate.rb b/app/models/spree/tax_rate.rb
index f0a7be6e111..0bc308f971c 100644
--- a/app/models/spree/tax_rate.rb
+++ b/app/models/spree/tax_rate.rb
@@ -15,7 +15,7 @@ def validate(record)
 module Spree
   class TaxRate < ApplicationRecord
     acts_as_paranoid
-    include Spree::Core::CalculatedAdjustments
+    include CalculatedAdjustments
 
     belongs_to :zone, class_name: "Spree::Zone", inverse_of: :tax_rates
     belongs_to :tax_category, class_name: "Spree::TaxCategory", inverse_of: :tax_rates
diff --git a/config/application.rb b/config/application.rb
index c4f609a4a43..175f191f459 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -34,14 +34,6 @@
 
 module Openfoodnetwork
   class Application < Rails::Application
-
-    config.to_prepare do
-      # Load application's model / class decorators
-      Dir.glob(File.join(File.dirname(__FILE__), "../app/**/*_decorator*.rb")) do |c|
-        Rails.configuration.cache_classes ? require(c) : load(c)
-      end
-    end
-
     config.after_initialize do
       # We need this here because the test env file loads before the Spree engine is loaded
       Spree::Core::Engine.routes.default_url_options[:host] = 'test.host' if Rails.env == 'test'
diff --git a/config/initializers/spree.rb b/config/initializers/spree.rb
index 321d7a93513..f17d1d83a7f 100644
--- a/config/initializers/spree.rb
+++ b/config/initializers/spree.rb
@@ -8,15 +8,6 @@
 
 require 'spree/core'
 
-# Due to a bug in ActiveRecord we need to load the tagging code in Gateway which
-# should have inherited it from its parent PaymentMethod.
-# We have to call it before loading the PaymentMethod decorator because the
-# tagging code won't load twice within the inheritance chain.
-# https://github.com/openfoodfoundation/openfoodnetwork/issues/3121
-Spree::Gateway.class_eval do
-  acts_as_taggable
-end
-
 Spree.config do |config|
   config.site_url = ENV['SITE_URL'] if ENV['SITE_URL']
   config.site_name = ENV['SITE_NAME'] if ENV['SITE_NAME']
diff --git a/lib/spree/core.rb b/lib/spree/core.rb
index 0b766960d1d..d737c6e7da8 100644
--- a/lib/spree/core.rb
+++ b/lib/spree/core.rb
@@ -47,7 +47,6 @@ def self.config
 require 'spree/core/delegate_belongs_to'
 require 'spree/core/permalinks'
 require 'spree/core/token_resource'
-require 'spree/core/calculated_adjustments'
 require 'spree/core/product_duplicator'
 require 'spree/core/gateway_error'
 
diff --git a/lib/spree/core/calculated_adjustments.rb b/lib/spree/core/calculated_adjustments.rb
deleted file mode 100644
index 3e06a951797..00000000000
--- a/lib/spree/core/calculated_adjustments.rb
+++ /dev/null
@@ -1,80 +0,0 @@
-# frozen_string_literal: true
-
-module Spree
-  module Core
-    module CalculatedAdjustments
-      def self.included(klass)
-        klass.class_eval do
-          has_one :calculator, class_name: "Spree::Calculator", as: :calculable, dependent: :destroy
-          accepts_nested_attributes_for :calculator
-          validates :calculator, presence: true
-
-          def self.calculators
-            spree_calculators.__send__(model_name_without_spree_namespace)
-          end
-
-          def calculator_type
-            calculator.class.to_s if calculator
-          end
-
-          def calculator_type=(calculator_type)
-            klass = calculator_type.constantize if calculator_type
-            self.calculator = klass.new if klass && !calculator.is_a?(klass)
-          end
-
-          # Creates a new adjustment for the target object
-          #   (which is any class that has_many :adjustments) and sets amount based on the
-          #   calculator as applied to the given calculable (Order, LineItems[], Shipment, etc.)
-          # By default the adjustment will not be considered mandatory
-          def create_adjustment(label, adjustable, mandatory = false, state = "closed", tax_category = nil)
-            amount = compute_amount(adjustable)
-            return if amount.zero? && !mandatory
-
-            adjustment_attributes = {
-              amount: amount,
-              originator: self,
-              order: order_object_for(adjustable),
-              label: label,
-              mandatory: mandatory,
-              state: state,
-              tax_category: tax_category
-            }
-
-            if adjustable.respond_to?(:adjustments)
-              adjustable.adjustments.create(adjustment_attributes)
-            else
-              adjustable.create_adjustment(adjustment_attributes)
-            end
-          end
-
-          # Calculate the amount to be used when creating an adjustment
-          # NOTE: May be overriden by classes where this module is included into.
-          def compute_amount(calculable)
-            calculator.compute(calculable)
-          end
-
-          def self.model_name_without_spree_namespace
-            to_s.tableize.gsub('/', '_').sub('spree_', '')
-          end
-          private_class_method :model_name_without_spree_namespace
-
-          def self.spree_calculators
-            Rails.application.config.spree.calculators
-          end
-          private_class_method :spree_calculators
-
-          private
-
-          def order_object_for(target)
-            # Temporary method for adjustments transition.
-            if target.is_a? Spree::Order
-              target
-            elsif target.respond_to?(:order)
-              target.order
-            end
-          end
-        end
-      end
-    end
-  end
-end
diff --git a/spec/lib/spree/core/calculated_adjustments_spec.rb b/spec/models/concerns/calculated_adjustments_spec.rb
similarity index 98%
rename from spec/lib/spree/core/calculated_adjustments_spec.rb
rename to spec/models/concerns/calculated_adjustments_spec.rb
index 218ef63c92a..b024c679e40 100644
--- a/spec/lib/spree/core/calculated_adjustments_spec.rb
+++ b/spec/models/concerns/calculated_adjustments_spec.rb
@@ -5,7 +5,7 @@
 # Its pretty difficult to test this module in isolation b/c it needs to work in conjunction
 #   with an actual class that extends ActiveRecord::Base and has a corresponding table in the DB.
 #   So we'll just test it using Order and ShippingMethod. These classes are including the module.
-describe Spree::Core::CalculatedAdjustments do
+describe CalculatedAdjustments do
   let(:calculator) { build(:calculator) }
   let(:tax_rate) { Spree::TaxRate.new(calculator: calculator) }