Bypass validation of shipping methods zone on checkout

app/models/spree/shipping_method_decorator.rb

@@ -53,6 +53,14 @@ Spree::ShippingMethod.class_eval do
     spree_calculators.send model_name_without_spree_namespace
   end
 
+  # This is bypassing the validation of shipping method zones on checkout
+  # It allows checkout using shipping methods without zones (see issue #3928 for details)
+  #   and it allows checkout with addresses outside of the zones of the selected shipping method
+  def include?(address)
+    return false unless address
+    true
+  end
+
   def has_distributor?(distributor)
     distributors.include?(distributor)
   end

spec/factories/shipping_method_factory.rb

@@ -41,5 +41,6 @@ FactoryBot.modify do
   factory :shipping_method, parent: :base_shipping_method do
     distributors { [Enterprise.is_distributor.first || FactoryBot.create(:distributor_enterprise)] }
     display_on ''
+    zones { |a| [] }
   end
 end