[cstejerean]

I would bet it's more expensive than Stripe. AmEX is known for being much more expensive than Visa and MC, and I wouldn't be surprised if Stripe loses money on every AmEx transaction (but makes it up on other cards).

[SyneRyder]

Stripe used to charge a higher fee for AmEx cards, until August of this year. (Or at least, they did in Australia.)

[tedunangst]

This would imply one might negotiate a better rate with stripe by promising to never send Amex their way.