I've also recently had this where I put in my name/wizard # and an AWD that's not my corporate code. It shows price X all the way through the booking pages, but when it confirms, it jumps to a higher price than X. Upon calling, they said "you're only eligible for your corporate rate, no others." I tried to explain "this is a personal rental, not business" and they didn't care. I had them remove the AWD associated with my profile and it worked. Now I just need to put in one whenever I make a booking, work/personal/etc. Works just fine.
Also, one can just put in the Costco AWD when booking on avis.com instead of using the Costco site.