fix nil cost_price + operator bug

app/models/spree/line_item_decorator.rb

@@ -34,7 +34,8 @@ def options_text
     end
 
     def cost_price
-      variant.cost_price + ad_hoc_option_values.map(&:cost_price).inject(0, :+)
+      #due to the fact that cost_price can be nil we must convert it into bigdecimal (to_f will make nil->0 without losing precision in case of value)
+      variant.cost_price.to_f.to_d + ad_hoc_option_values.map(&:cost_price).inject(0, :+)
     end
 
     def cost_money

app/models/spree/order_decorator.rb

@@ -37,7 +37,7 @@ def contains?(variant, ad_hoc_option_value_ids, product_customizations)
       end
     end
 
-    def merge!(order)
+    def merge!(order, user)
       order.line_items.each do |line_item|
         self.add_variant(line_item.variant, line_item.quantity, line_item.ad_hoc_option_value_ids, line_item.product_customizations)
       end

config/locales/en.yml

@@ -1,5 +1,6 @@
 ---
 en:
+  add: "Add"
   required: "Required?"
   price_modifier: "Price Adjustment"
   price_modifier_type: "What price modifier strategy does this option type use?"
@@ -46,4 +47,4 @@ en:
   customize: 'Customize'
   reset: 'Reset'
   position_within_product: 'Display order relative to other Ad Hoc Options (low numbers are displayed first)'
-  selected_by_default: 'Selected by default?'
+  selected_by_default: 'Selected by default?'