Originally Posted by AutoSlash

The additional driver won't be a problem from the perspective of "finding out", plus as mentioned earlier, just make the backup reservation for a little later and then cancel it from your phone before you check out.

The age could be a problem though. They may let you add a driver over 20 or 21, but they definitely won't let you add an 18 or 19 year-old. That I know from experience. You may just have to suck it up and decide whether you want to take your chance with UC or pay more to guarantee the vehicle you want.