This is a very interesting short article on charging interest on Accounts Receivable in the state of California and how the CA Supreme Court ruled in favor of the seller of supplies who charged an interest rate 1.5% per month (18% per year) which is higher than CA usery laws cap at 10% . Read why in the linked article below…

http://www.calitigationblog.com/2011/03/articles/contracts/interest-provisions-are-enforceable/

Another interesting article. http://answers.google.com/answers/threadview?id=32733

Since this doesn’t seem to be a clear cut matter in California, you should seek the advice of an attorney in this matter.