As a follow up, screenshot of part of credit card current charges statement showing new E-ticket numbers generated by UA for ancillary charges (in this case, an upgrade co-pay). Both are different than E-ticket number on itinerary (which stays the same) to which these were applied. Should be able to get some sort of receipt based on researching the E-ticket number corresponding to the OP's $125 charge. The difference between my case and OP's is that both $550 charges showed up on my updated current itinerary receipt (which led me to look at charge card listing)

UA managed to charge me twice for my upgrade to PF LHR-SFO. Took only 30 seconds for agent to see the error and reverse.

I took the time and sent a long email to 1kvoice with the timeline of all my transactions, interactions with United, contact attempts, and copies of all receipts on Friday, 9/1.

I received two receipts yesterday, 9/4 - one that was a carbon copy of the ones I already had, then a second one that was correctly priced and finally showed the $125.

No actual correspondence or apology, but I'll take what I can get.