Check your local law. In some jurisdictions, you can charge interest, or penalties. You can be gentle about it - give fair warning, reminder that intrest is starting to accrue, etc. But customers don't generally want liabilities to increase, so will prefer to pay before extra costs are incurred.

Here is one company's experience (UK focused): https://www.revk.uk/2026/03/late-payments.html

It depends a lot on your relationship with the customer as well, I guess. Some may get butthurt about it, for others, your relationship is with a person in a different dept to the people organising the payments, you can send interest notices to the finance dept without worrying the person who wants your services.