If you pay manually, you probably don't want the direct debit setup

Companies have to give 10 working days notice of how much they'll be collecting. Amex claim that if you pay between when they give the notice (typically with your statement) and the direct debit date, they can't change it.

I think that if you paid the whole lot off, amex would be OK under the direct debit rules to take nothing, but they don't seem to agree on that. What isn't ok would be for you to pay half the bill, then expect them to only take the other half after they've notified you how much they'll be taking. It's a pain though...