We normally discuss the service we offer with our prospect clients over a period of weeks, after which we agree upon a certain estimate.
Each service is individually priced so the normal shopping cart solution does not work here. Instead, we bill customers and allow them to make partial payments that are credited to the pending invoice until fully paid.
In special cases, we just take money in advance that is later credited to their particular future invoice, so the system must be able to credit the funds to the customer's account. In this last case, customers will have to have a form they can fill out and enter the amount to be paid themselves. In addition, this form must be made accessible without the need of registering, that is, accessible from the free area to allow anyone to pay, not just registered clients.
Does anyone know if such system exists (or close) and if there are any scripts out there behaving this way with some backend features (accounting, invoicing, support ticket systems, print capabilities, import/export data, payment gateway integration, etc...)