I just spent fifteen minutes tracking down some FIA Business Card Services contact information. Here's what I've found:
Phone Number: (existing customers)
Business Card PO Box 15710 Wilmington, DE 19886-5710
I am not aware of how FIA Card Services handles their payment services, so I offer no guarantee to the accuracy of this information. I'm mainly posting it here as a reference for myself, and to encourage FIA to make their contact information more accessible to their customers. I encourage you to contact FIA Card Services directly if you have any questions. Good luck!
UPDATE: I got ANOTHER %2.9 promotional rate offer from FIA today, but since I already took advantage of it, this one will get the shred. I mention this because the return address on the envelope was similar, but different than the one I posted above. It is:
FIA Card Services P.O. Box 15078 Wilmington, DE 19850-5078
Since these two post office boxes are different, I would definitely try and contact FIA and double check where to send payment. While it is possible that FIA would eventually credit payment to the appropriate account, I wouldn't want any delay to affect my payment arrival and posting date, lest it result in an overdue payment.