Adding offline orders to an online Payment modules

Hello everyone, this is my first post.

I am working with a Dry Cleaning company that wants a site with a payment module that allows customers to add items to their monthly account and for the Store Managers to add offline orders to the custmers online account for payment at a future date. The Company has customers who run a 'tab' that they pay off on an ad hoc basis according to their status.

Any suggestions about a suitable payment module?

Lavers