I talked to Amex customer service on the phone and the reps. there didn't seem too sure whether charges through officialpayments.com wouldn't be eligible. But they didn't see anything that would rule it out.
The stuff they exclude are things like cash advances and the annual fee of the card.
But just to make sure I logged in and got a written statement saying the charges would count and would also count towards the 10,000 MQM mileage boost.
Also got an email from Officialpayments.com saying they process multiple charges. Might have to do it over the phone rather than through the web site.
I also requested a credit increase from Amex but they said 7-10 days before an answer!
"Eligible" for exactly what? If it is AmEx MR, airlines miles, hotel points (e.g., SPG), or the like, I don't know of any reason to doubt it. Chase gives mileage credit for a tax payment through officialpayments.com the same way that it does for other charges, and one gets mileage credit for the "convenience fee" payment in the same way too. What I wonder is whether a card that rebates 1 or 2% cash treats these tax payments the same way as it does payments for good and services, when the merchant/provider is paying a few percent vigorish on the transaction to the card issuer. I would want to be very, very sure that AmEx or whichever card issuers was involved would not treat it as a cash advance, tacking on those fees, which could make it a very costly way to get miles or points.