You should have received an email with proof of payment that shows bitcoin converted into USD shortly after you purchased them. Simply print it and show it to them.
EDIT: I have noticed my most recent invoice from BITMAIN only shows BTC for payment and no USD conversion. Maybe the BTC will be sufficient.
They could care less what the invoice looks like. Make your own invoice, slap the Bitmain logo on it and print it. Obviously throw a little common sense in the mix. Send them all back to CHINA!
Just take the Bitmain invoice and translate the BTC value by adding an USD value next to them, and give them the exchange rate somewhere on the page. But i don't think this is something you can do at the door. At least it is not over here. You have to fill forms and crap and request a re-evaluation and ask for a refund. Heh.