Well, I fell for the "tip" mentioned in the (now removed) blog post titled:
"
How To Earn 3x Points (Or More) For Paying Your Taxes By Credit Card"
I have only myself to blame for this of course.
Just in case anyone else is tempted by the idea that you could get 3x for paying taxes with Amex Checkout, no tax provider qualifies for the bonus.
Yes, a tax provider is mentioned on the bonus page (some lack of clarity from Amex there) but not all vendors on the bonus page qualify for the bonus - they just all accept Amex Checkout.