If you are trying to use a code you are not entitled to use, and you are asked for ID to prove your entitlement, you are going to face paying full rates for your rental. So backup would be a rate that is lower than full price but more than that 'great' rate you got with a coupon/code you aren't entitled to use.
There is no way I am using any code that I am not sure I am entitled to. I would rather pay more and be sure of the rate I was getting than to be hugely surprised at the counter with having to pay full rate.