You might want to be more specific whether you want to be billed by the hour or whether you sell a packaged good. If the latter is the case (as I assume it to be your idea), you need to be VERY specific of your deliverables, otherwise you might be held reliable for things you did not even think are in the specs, but some lawyer can arguement that it is implicitly there. Take this with a grain of salt though as I'm not sure about the law in the US.
