A few things:

A collective noun is a noun that appears to be singular, but can take the plural, no? Like, "the team is all here" could also be expressed "the team are all here." So, FiberB, I appreciate the concurrence, but I have to hand some of it back to you (in an entirely non-peevish way, I assure you).

It seems to me that one account, which is to be paid, is an account payable, and more than one of these would be accounts payable, and that the department that handles these would be the Accounts Payable Office or something. I understand that usage seems to have gotten us to the point where one of them is an accounts payable (that indefinite article can't be right...), but how and why?

Looking back at the replies so far, it seems that the idea of the double plural is to describe multiple sets of accounts payable - but why wouldn't this also be accounts payable?

On a funny note, related to accounts payable and language:
http://www.theonion.com/archive/archive_kornfeld.html