Custom shipping gateway

gateway.rb

module ActiveShipping
  class MyBuild < ActiveShipping::Fedex
    def find_rates(*args)
      results = super
      # some code to customize the results
      return results
    end
  end
end

initializer.rb

unless Rails.env.test?
  Workarea.config.gateways.shipping = ActiveShipping::MyBuild.new(
    login: '999999999', 
    password: '7777777', 
    key: '1BXXXXXXXXXxrcB', 
    account: '51XXXXX20'
  )
end

I'd suggest writing unit tests to cover the custom gateway's functionality.