Who do you think you are? A bank?
Leave the interest charging game to the banks.
Charging interest on overdue invoices gives your clients 2 choices:
- Pay the invoice
- Not pay the invoice (and start accumulating interest which they are pretty sure they can weasel out of with a phone call I they can be bothered)
Why give them 2 choices when you really want them to only take option 1 to save you (and them) all the extra hassle?
Focus on your goal: To get paid.
Charging interest on overdue invoices is a distraction from that goal.