Will, sorry for the late reply. You’re going to have to have a word with your dealer. It’s basically as Lord Muc said, money taken off your AP. What I would do in your position is mention the offer and say Marshalls have accepted it and a few other people you know have received this and to save all the hassle can I be eligible. Work your magic with him/her.
If they refuse, but you still want the offer and you want to order somewhere else, i’d cancel but not sure about your deposit as I haven’t come across cancelling a order. You are within your right to cancel then the dealer in the end will most likely call you back and say yes to you but get it in writing! My dealer added the discount, deposit taken on the order and sent it me. Hope it goes well and do tell us how it goes!