Late Inovice Fees

In the State of New Jersey what is the maximum I can charge for late fees on an overdue invoice ? I also notice there is interest late fees or just a late fee. I really just want to have something legally put in place for my clients who drag their feet in payment. I have a web design business. Most of the invoices are small but nevertheless I need to get paid in a timely manner. I've seen other design firms charge 1.5% or more of the total invoice at 15 days late, then again every 15 days. What should I do?
Thanks

If you don't have a contract that specifies when payments are to be made and the fees for late payments, it's hard to justify *ANY*.

Generally, you put a NET30 clause in the contract, that says they have 30 days to pay or else you start accruing interest. NJ is pretty liberal with this ,you can get up to 50% APR there. HOWEVER, in a web business you need to make sure you're not running afoul of rate limits for out of state customers.

You may wish to offer a prompt payment discount.

Not that I understand it but does this fall under the state usury interest limits ? Would it be handled the same way as loan interest ?