<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Bidkat:
Once the convenience fee (2.5%) is paid, is the tax amount treated as a purchase -- i.e., no interest accruing until payment due? If so, you're also getting up to 90 days free float with Amex, depending on the timing. If you're dealing with a large amount, it is even possible to request a change in your CC's due date to maximize the float period. Most CC's will let you do this one time.</font>
In fact, this is exactly the situation in my experience. I've paid taxes using a MasterCard and American Express card, and in all cases, the transaction is treated as a purchase. In fact, this is why you're paying the convenience fee to the third-party processor: they must somehow recoup the discount rate they pay the card issuer. These plans would make little sense if they were treated as cash advances, as the two major benefits of them are a) interest-free grace period, and b) mileage accrual, neither of which would be benefits if the transactions were treated as cash advances.