Is there really only a net cost of 0.49% to pay with the credit card? If that were the case, then when was earning a 6% or higher return on there money (not many such opportunities these days, except perhaps in loansharking), they might actually come out ahead with the float. I doubt that Schwab and the bank issuer of the card have set things up in such a way as to permit that.

When Chase is giving out airline miles, it treats a tax payment made subject to that 2.49% as it does other purchases, that is without anything like a cash advance fee. With a credit card that rebates money rather than gives you miles or points, I wonder if they too treat tax payments the same as ordinary purchases with the card rather than impose a cash advance fee in addition to the 2.49% "convenience fee" that Officialpayments imposes for handling the tax payment. Take care lest this cost you substantially more than you anticipated.

Also, you may not be able to use a combination of credit cards to make tax payments through Officialpayments. There are money laundering rules and regs that affect these transactions, and I was told last year that one could make only one credit card payment per Social Security/TIN.