Amex/Authorize.net AIM acceptance issue

Hi all, I'm having an issue getting Amex cards to go through. Visa and Mastercard are working fine, and we've already accepted several orders on those cards.

I DO have Amex enabled in Admin > Config > Credit Cards, and the Authorize.net account is set up to accept Amex, so I know that's not the issue.

My project manager got in touch with the client to discuss the issue - here's what he said:

"We had out client call Chase Paymentech and they told them to contact Amex. The client then called Amex who told them that you had to program this amex merchant id (#####) into the site for amex to be accepted."

I've never heard of card-specific merchant IDs, and I didn't find any settings in the Admin section that looked relevant. Has anyone heard of this?

My site is:

http://www.vkoonyc.com

Currently running 1.3.9h.

Thanks for the help - cup of coffee purchased!