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Incase people are interested AMEX apparently allows you to pay a amount more then you owe. I owed aprox $470 and I selected the option of other amount and was allowed to put in $600. Now i did have a pending sale of $215 but it hasnt posted yet.
Yeah, I did this as well. I had a rental car for a week and wanted to offset the hold they had on the car. I called Amex first and they said, "no problem." There have been no adverse effects for me.
I overpaid them once through my bank's online system. Amex just cut me a check.
I ran a negative balance on my Delta card for a couple months, then called Amex and asked them to credit one ove my other accounts--which they did with no hassle.