[work in progress]
. PostalService now inherits from Spree::ShippingCalculator
  . The commit-e09735c622326b2f2e14ee2c3de212145eb83894 on spree-core forces Shipping calculators to inherit from Spree::ShippingCalculator only. Otherwise tthe calculator doesn't show up in the available calculators list. [Ref: https://github.com/spree/spree/commit/e09735c622326b2f2e14ee2c3de212145eb83894]
  . There is also a corresponding change in directory structure and in engine.rb
. PostalService#compute changed: accepts a Stock::Package object as argument instead of Order or LineItems [Tests yet to be changed]
. PostalService#available? changed: accepts line_items as arguments instead of an Order [Tests yet to be written]