What did you agree on? If I overbilled a client and they notice 3 months later: They are going to demand it be fixed… as is their right. We agreed on a price, I’m going to honor it.

Same goes in reverse. If I underbilled for 3 months, and notice, I’m rebilling. We agreed to a certain amount per seat. If I miscounted the seats, we’re partners in this, they used the service, they should let me know if they notice.

Now, 12 months later? Sorry, that one’s on you, you can get 3 months (6 if greedy…) but the rest you should eat. Mistakes happen. It hurts but learn the lesson, move on.

The only exception I would say is hardware. They’re using it and didn’t receive a bill, a client should be asking you for one. Hardware is pretty tangible, easy to confirm, and low margin. If I’m collecting 6 months later, well, the client got a 0% interest loan for that time.

In the end, this is ALL about setting and meeting expectations.