You have a reservation and ticket with receipt. I wouldn't mess with it.

United's computers can be challenging in the best of circumstances. I couldn't imagine what they have to do to handle a non-traditional payment method such as this.

Just leave it be. Best case, they never charge you. Worst case, you have an itinerary and receipt, and they will honor it with a bit of a fight.

But finding someone both knowledgeable and empowered to push something like this through in an organization as dysfuntional as post-merger United, is practically impossible.