I'm with BrianPan; don't ever tell on yourself until you're certain something has gone wrong.
Years ago I had a similar experience with AMEX and I did the same as you, I called customer service and 'fell on the sword'. They were nice and noted my account although they weren't showing the returned check yet. I even overnighted a cashier's check to replace the 'bounced' check... I thought I had everything covered. I never counted on AMEX resubmitting the check two more times to my bank (of course, by now I had used the funds to purchase the cashier's check). I eventually had to stop-payment on the original check, which looked *much worse* than a bounced check. It was over a week later before AMEX showed I had a returned payment, which of course had been covered by the casher's check days earlier. Only then did AMEX charge me a returned payment fee.
I would have saved myself numerous fees, not to mention my "up-until-then-flawless" reputation with AMEX if I had just let the dust settle before I freaked and tried to 'proactively' fix everything, since everything would have fixed itself and (other than the original NSF bank fee) nobody would have been the wiser. Lesson learned...
