When to use "payables" vs. "payable"?

When do you use the term "payables" and when do you use "payable" in terms of accounting? Here's my thoughts and please correct me if I'm wrong: If you are referring to the department as a whole you would say "Accounts Payable". However, when talking about the items themselves you would refer to them as "Payables". The reason I ask is, a girl I used to work with always insisted upon saying "Accounts Payables". It drove me crazy because I just don't think that it is proper grammar to have both words pluralized. What do you think?