You might consider simply using your bank/credit union's own online bill pay service to make payments, and not put your checking information in her AMEX file.
I have a relative who's AU on a couple of cards, and I've made a couple of other online payments for him, but only from my own account. I trust him implicitly to not intentionally do something like make a 10,000 payment instead of a 100.00 one, and to not intend to select the wrong account if he were to have a few to choose from for payment - but I also know how often I've made idiotic errors when I'm not paying close attention, and I'd never want that burden to be on him. I'm excruciatingly comfortable with technology; he's not - and yet I still make mistakes.
By paying from your own bank/CU, you're ensuring that nobody else can inadvertantly take money from your account, or that no payments will be made when you don't have the balance on hand to cover it. Any errors will be your own.
) That one took a little bit of explaning to fix. If there's stupid mistakes to be made... I've made 'em.