A very powerful solution that we suggest is Modern Bill. Hope this helps.
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Integrated billing automation software is not something we sell for a profit. We usually include it free with our reseller plans. However, the one that makes the most profit is Reflexion spam protection. We offer it to clients, resellers and other hosts who want to offer it to their clients. Spam is the biggest problem our customers have and they are happy to pay for a good service that saves them the effort of deleting spam or having to build their own filters.
Most of the other programs, customers want for free because cPanel w/Fantastico provides so many free scripts instead of upselling them to end users like other control panels do. This crushed the upselling business model years ago. Where upselling scripts to customers and incrementally increasing revenue based on their needs, was used to charge very high prices for hosting. It was a much more profitable business model that wasn't based solely on the size of your packages.
As the others have mentioned intergrated billing systems is a great feature to offer to your customers. Aswell as e-commerce tools to online store owners such as merchant accounts. However, often alot of providers make the mistake of selling. I say bundle it, do not upsell. At the end of the day, your pricing on the package will slightly increase and the customer feels at ease with features being bundled rather then "addons" that they must purchase. In HF's experience bundled packages sell better.