I have a service company who manually bills their customers

I have a service company who manually bills their customers but would like to put a PayPal option on their site.

They need for the ability for the customer to input their invoice number and amount being paid onto a form that submits to PayPal for payment...

I've found plenty of ways to integrate PayPal options into Wordpress with fixed payment amounts or shopping carts, but this situation is unique in that we need the customer to input the amount they are paying on their invoice or account dependent upon the amount of their invoice. And they want this to occur without the customers having to have membership on the website.

Any suggestions on where to look or how to get a Plug-in for such a task.