diff --git a/app/models/spree/calculator/shipping/usps/base.rb b/app/models/spree/calculator/shipping/usps/base.rb
index c8486d3466..240f2f38bd 100644
--- a/app/models/spree/calculator/shipping/usps/base.rb
+++ b/app/models/spree/calculator/shipping/usps/base.rb
@@ -3,7 +3,7 @@ module Calculator::Shipping
     module Usps
       class Base < Spree::Calculator::Shipping::ActiveShipping::Base
 
-        SERVICE_CODE_PREFIX = {
+        SERVICE_CODE_PREFIX ||= {
           :international => 'intl',
           :domestic      => 'dom'
         }