Is your current letter strategy saying the same thing each time or does it have escalated intent in the verbiage? If you want to make the most of your letter series, they should look and sound different.
Letter 1- Should be sent about a week before the invoice is due and include a copy of the invoice. This way, if they lost or didn't receive, the invoice for the first time, you are automatically resending.
Letter 2- Should be sent a week after the invoice is due, notifying the customer you haven't received payment. If they want to avoid late fees, maximize discounts with you, etc. they should process payment immediately online. If you are sending this via email or pop-up message on their portal, the link should be included to process payment. Automatically offer the customer the discounted rate if they process payment before end of business day. If you don't have a payment portal, honor the discounted pricing with payment process via phone that day.
Letter 3- Should be sent approximately 10 business days after letter 2 if payment still hasn't been received. You should be mentioning possible service interruption, loss of discounts and/or late fees if unresolved. You want it to be clear that the account is in jeopardy of service interruption if not resolved within 5 business days.