> Matt and I came to this project after we built similar internal billing systems at previous jobs and we realized how error-prone these systems can be—one incident might have even undercharged a client by a few million dollars!

This honestly doesn't lend me the confidence in your solution that I assume you think it does ;P. "We've done this before, and dude... we failed at it SO BAD, you just don't understand: this is HARD. So, instead of doing it yourself, you'd be much better off out-sourcing that effort... to us." :( It is a subtle difference, I admit, between versions of this pitch that work and the ones that don't, but this one just didn't work for me.