I've gotta go with Pat on this one. Yes, the OP should have called the number on the letter, but to him/her, United is one company. Despite what we FTers think, not everyone is an expert on the inner workings of airlines. If I had been the OP, I might have also thought that a general customer service number could help, too.

From a business standpoint, it would make sense for UA to just refund the $15. I would start by calling them. If you get no satisfaction, call the CC company.

Good luck,
Mike