It's just an invoicing program. There are lots of those. Intuit's is widely used. Oracle is also in that business. There's "Anytime Collect", "Zencash", "Esker", "Celtrino". and many others.<p>The more advanced players support electronic data interchange (EDI), where your accounts receivable system connects to the buyer's accounts payable system. Many large companies require EDI for suppliers who generate a lot of invoices, so they aren't re-entering invoice data into their own systems.[1] Any new system should have EDI interchange with at least all Fortune 1000 companies.<p>There are "gateway" companies which handle talking to large numbers of other companies.[2] Once you get this all working, your invoice goes out to the gateway, the gateway formats it and sends it to the paying company, the paying company's systems validate the bill, and they do a funds transfer to your bank account, which is matched to another EDI transaction indicating payment. For most routine transactions, there's no human intervention.<p>Make that all work for small/medium businesses, and you have a unique product.<p>The API is "dead simple" because it doesn't handle any of the hard cases.
Like "We ordered 1000 but 200 didn't arrive. We're paying for only 800".<p>[1] <a href="http://www.aepedi.com/apay.htm" rel="nofollow">http://www.aepedi.com/apay.htm</a>
[2] <a href="http://www.b2bgateway.net/" rel="nofollow">http://www.b2bgateway.net/</a>