Filed a DOT complaint against UA for the overcharge. They left me a voicemail saying that the charge was correct, and the receipt shows the whole ticket price regardless of the form of payment (in other words, they didn't read my letter, detail, and copy of credit card charges I attached).

A couple days later, UA left another voicemail saying that it was an unintentional technical glitch, and they'll refund me the 93 cents x 2 tickets or whatever it was. I wonder if they sent their proposed response to DOT who may have rejected it as not addressing the concern.