Accounts payable and accounts receivable are standard business terms. They are group singulars (if that approximates the proper grammatic term) and would not be taken to refer to any specific collection of accounts. A single account payable is not anything I have ever heard but then I'm not an accountant so I wouldn't really know. However, if you had some specific collection of individual members of the category accounts payable it would seem to me only logical that you would take the term accounts payable and tack an s on the end of it giving you accounts payables.