American Express Membership Rewards - Any comments about paying Fed estimate w/AMEX card?

I go this e-mail from AMEX about paying estimate taxes with card. On 50,000 it would be $1,250 fee to Official Payments. Any other fees? How about intesest when it posts to AMEX? AMEX charge interest? Please advise.

There are dozens of archived threads discussing this. It will post as a purchase, not a cash advance. If you time it right you can get a 50-55 day float.

But at $.025/mile, you'll have to decide whether it is worth it.

Thanks. I also see I can buy the MR points but the "float" looked like a bonus if no interest is involved.