add custom fields on order serializer

override_serializer.rb

# Override the order serializer, adding the needed attributes:

class MyOrderSerializer < Spree::Wombat::OrderSerializer
  attributes :shipping_instructions
end

# make sure to update config/initializers/wombat.rb to include the new serializer:

config.payload_builder = {
  "Spree::Order"  => {:serializer => "MyOrderSerializer", :root => "orders"},
}