I am tired of extending credit (verbal without interest) to my customers. Where do I look for software and laws to begin charging interest?

Well you know that if your customers are individuals and not businesses, that consumer credit is a highly regulated affair.

I can only remember one customer in eight years that financed consumer debt in-house, but I have no idea what he used. I don't know any particular software vendor, but this is a good place to start looking:

http://directory.google.com/Top/Computers/..._and_Collection

That is the Google/Dmoz directory for business credit and collection

Good luck