app/models/spree/payment_decorator.rb
##
# Amazon Payments - Login and Pay for Spree Commerce
#
# @category Amazon
# @package Amazon_Payments
# @copyright Copyright (c) 2014 Amazon.com
# @license http://opensource.org/licenses/Apache-2.0 Apache License, Version 2.0
#
##
Spree::Payment.class_eval do
scope :amazon, ->{ where(source_type: 'Spree::AmazonTransaction') }
end