To be honest, I would trust HealPay more if you charged a fee. As others have mentioned, the fact that you don't charge makes me question where you will make mo

To be honest, I would trust HealPay more if you charged a fee. As others have mentioned, the fact that you don't charge makes me question where you will make money and how you will survive. If I'm going to trust your site with my invoicing, I want to be sure you will stick around. Invoicing is an interesting domain because your users are receiving money as a result of your service-- thus charging a fee only means slightly less money coming in (so cognitively it's treated as a smaller gain than an actual loss). My suggestion would be to offer a free account that gives a taste of your service but leaves your users wanting more. FreshBooks is a good example-- you can only have three clients before you need to pay. Other options might be only paying if an invoice is above a certain amount. Finally, a huge reason I use FreshBooks over email or my previous company's proprietary system is because I can give my clients a clean, professional interface where they can log in, view their invoice, and pay it online. So integration with Authorize.net, Google Checkout, (and I suppose PayPal, though I dislike them) are key. One thing you can do right away that is a step above FreshBooks is to allow authenticating against an external url using a REST API. This would allow me to give my clients ONE password to use on their sites as well as for billing. Hope that helps.