Check your state laws on maximum penalty. We choose not to charge maximum allowed by law of 24% - we charge 18% per year, or 1.5% per month on outstanding balance. I used to charge in addition a $5.00 late fee, we just upped it to $10.00 this year. Got tired of the same people stringing me out, so I figure if I'm going to keep their business (many of them are right next to or down the street from other clients) then they'll pay for the privilidge to use as a credit institution though we are not.
Set your terms up in your contract and then stick to them. You did the work and should expect to be paid on time. Explain to them that you can not do any more work for them until their account is brought up to date. If you're a reliable vendor for them and they value your service then they'll pay you on time if you request. But since you're not the squeaky wheel, you're not going to get the grease.
Lawn Lad, Inc.