If your client is a personal contact with whom you have had prior contact, you can include a personal letter with the invoice. For their convenience, you have included a self-addressed stamped envelope for their payment. Make it as easy as possible for them to pay. The self-addressed stamped envelope obligates them, but in a polite way. Clients may not respond to your first invoice letter.
Most companies send several, with increasing urgency in tone.
If time and budget allows, this is your next step. If you provide an ongoing service, you could warn them that their service will be discontinued if payment is not received. There are a few circumstances in which this is illegal. Check your local and state commercial laws to confirm. Visit or Call.