Can customers pay service invoices online?

My core business is the technical service industry but I have an on-line store established for customers to purchase spare parts and have them shipped directly to their facility. Here is my question....

If I go to 'Joe Customers' facility and repair something, can the invoice be somehow put into Zen-Cart so the customer has the option to 'pay on-line'?

I have looked through the add-ons for invoicing and dont see anything matching that description. Does anyone have a suggestion?

Thanks in advance.

This might be what you're looking for: http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=230

I looked at that one, that one allows customers to be invoiced for purchases made on-line. I'm looking to put an invoice on-line for a customer service performed in the field.