I seem to remember that almost the very last option when paying with QR on their own website is to change the currency from that of the departure country to that of your card. But the website cannot do that until you have provided your card number as it uses that to detect the billing currency of your card, so it really is the last last option before you click to confirm the purchase. I seem to recall that the offer I had was about 2.9% off mastercard published rate that day so it was similar to what cards typically charge their customers for foreign transactions.

If that option does not present itself you might be able to purchase over the phone and pay in USD (especially if you call the US QR call centre) but there is probably a service charge for a call centre transaction.