If you can use a web-based invoicing and billing solution for businesses and web designers, check out CannyBill and get it at a discount, which enables users to create, send and manage invoices and accept payments online via credit card, as well as to sell your products and services using order forms that your customers can signup to and even link to live API's such as Plesk, Geotrust, Enom, Basecamp and many more.
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