I’m Guam based and I have had success selecting the pay at a city ticketing office option in order to hold the reservation for at least 24 hours, then calling united to pay for it with a credit card. When I tried paying for the reservation online, it cancelled the reservation when it noticed the discrepancy in billing address. I called the web support desk and they were able to reinstate the reservation and took my Guam based credit card.

Additionally, maybe as a last resort, have you considered buying secondhand electronic travel certificates from an electronic marketplace like eBay?