Any report back on this? In similar situation where I was considering booking through cars.united.com in order to get the TravelBank credit from using one of the UA credit cards, but before clicking on what appeared to be the final confirmation link, I noticed at no point had I entered which credit card I wanted to use.

I don't really want to wait in a line to pick up the car, especially when going through UAs car rental portal is already around $10 more expensive than booking direct through Avis, which will eat into the TB credit that I would get.