One of the provisions you must have in your promissory note is a Late Payment Charge. The wording will be something like this: “If any Installment Payment is not paid in full by Buyer within 15 days after the same is due, then Buyer shall pay a “late charge” of 5% of the amount of such late installment payment for each month or fraction thereof that said Installment Payment is beyond due date.”
The purpose is to convey to your buyer that you expect payments to be made when due, and if not, a penalty will be assessed and enforced.