Kill all the decorators kill them all

Just had a spree 1-3-stable promo decorator dejavu, I wouldn't recommend
it to anyone. We have pretty much this same code in spree/spree already

see https://github.com/spree/spree/blob/2a2034f63717ebd58883a53849c96c8fae75da4d/frontend/app/controllers/spree/orders_controller.rb#L76-L85