Gift Card Payment Issue - Incredibly Poor Support

I recently purchased a gift card from the United MileagePlus X App in the amount of $500. I used the card as payment on an apple.com order for an Ipad. I purchased gift cards in the exact amount of the order and proceeded to check-out. I was having the order sent via courier same day and was keeping an eye on it. After it not releasing through the ordering process for a while I called apple.com support and was told the total for the order was different after check-out with the applicable tax and $18.12 more was owed. I gave the support rep another gift card for that amount. After the courier window provided was missed I called back and was told there was an issue with the order and it was cancelled. I was told the funds would be put back on the cards in 24 hours. I have verified that the funds are back on the cards but I am now unable to use the $500 gift card when checking out on apple.com.

I have spoken to multiple Apple Gift Card support reps at length, all who assured me the card is good and active. They transferred me twice to two different phone sales reps who "should be able to assist", only to have them get the same error upon check-out that i experience when attempting to order online. Multiple gift card support reps told me they would escalate the issue and open up a boss case and the team would contact me within 24-48 hours, only to both times fail to get the follow up email that I was promised. The last rep I spoke with this evening claims to not have any record of my prior calls or boss cases that were opened and stated, "if there is no notes or record I'm going to have to assume that means there is nothing we can do about it". He directed me back to United for a refund. At this point it seems I may be out $500 unless United shines through.

Has anyone ever experienced a similar issue with using a gift card after it was used on a subsequently cancelled order? It seems to all stem from that order, yet Apple keeps saying everything looks good on their end. I love Apple products but seriously considering moving away after this horrendous customer service experience that has spanned over a week and a half and multiple hours on the phone and chat.