So I figured I'd report back after trying to buy the smaller Amex cards with my $100 one... But I stopped after work yesterday, and I got the $100 Amex with my Visa, no issues. But the coupon for the $5.95 off didn't go through. My account registered, and the fuel coupon showed up
The cashier didn't bat an eye when she told me I owed $5.95 after swiping the Visa.
Has anyone else used these coupons yet? It's still on my account... So I may try buying a $100 Amex with the $100 Amex, and then using it to buy the $25/$50. If it works, I guess I got 424 extra fuel points for $6 OOP... If I have trouble, I still might buy the Amex cards with my credit card, because it's on a promo for 3x points back at grocery stores through the end of the year (all of my other credit cards are always 1%).