[paulhart]

If you're building a product, is the billing component a part of your USP?

Really?

Odds are that it's not, and therefore you should farm out that work to someone who _does_ make it their job. There are reasons why companies implement software from Microsoft, Oracle, and SAP, including that it's better to reduce costs on things that don't differentiate you in your market (and it's nice to have someone to "pointedly talk at" when things go wrong).