I dont think I sign up for the flexible payment option at all...

Would set up the email alert now - being 7 days before the due day - that is the usual time I schedule payments on all my cards, just give some room to fix any possible system glitch or whatever.

AMEX never let me schedule payment BEFORE statement close - unlike Citi and Chase, they both let you schedule payments up to the full amount of current cycle, and not later than statement date - AMEX wouldn't let me do anything, and that makes me leaving the card home when we go on a very long trip, as I cannot count on internet access to schedule payment when and after statement close. Their loss.