It sounds like you should be fine, but I recommend calling the branch where you plan to pick up the car just to remind them/chat with them etc. This is typically the point where if you book the tiny car, the agent might chuckle a bit, but I don't take it personally. As long as they honor my reservation with the price I was guaranteed, equipment substitution, while annoying on occasion, is part of doing their job.

Also, another reason why it's a coupon: You can't use any other coupon, so your rate will be higher than it otherwise could have been.