My United Explorer Card has a Brazilian billing address, while my Am Ex card has a U.S. billing address (long story). Both paid out of US bank accounts. If I buy UA tickets for an upcoming trip with the UA card, I'd get double miles, but would have to pay in Reais. Exchange rate looks pretty good for the initial exchange of USD to Reais to calculate the ticket price (2.32 Reais per $1). Then there'd be a conversion back to USD for the cc bill. No foreign currency transaction fee. Offhand, it looks worth the risk that the exchange rate will change by time of final billing, since I'd get double miles. Naïve question: am I missing something?

With two biz class tix at $3800 pp, miles gained would be $7600 vs. 15,200. If I paid with Am Ex, I'd get 7600 points that could translate to 9120 Hilton Honors points.