[leknarf]

The US also has usury laws, which I found fairly surprisingly when I was just starting out. I previously thought usury was a strictly a historic crime, like adultery or eating bacon on the sabbath. But some people are very much offended by the idea of lenders charging too high an interest rate.

They actually become relevant for late fees, since you usually do want your fee to be punitive enough to encourage timely payments. The usual practice is set your fee to 1.5% per month or the highest percent allowed by law (i.e. just below usury if that's applicable in your state).

[fizx]

IANAL, so ask one first if that's your thing, but what I've done is offer 20% off if paid within 15 days. Set your billing rate higher to compensate.

[rmc]

Usury is a different thing to do with charging high interest rates on loans. Many places have limits. This is default late payment fees.