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AMEX won't care about the P.O. box as far as a mailing address.
You will have to give them your physical address as well if you get your
mail at a P.O. Box. Both of them should be on your reports to be verified.
Wish you best of luck, hope it turns out in your favor, let us know!
They probably couldn't give you an instant decision via computer because of your thin file and so they sent it to manual review. But since they are closed there wasn't anyone there to do the manual review so it'll most likely be getting done today by a person. You'll get a HP on your EX either today or tomorrow. Good luck