I was assuming you’d already sent the reminders etc out and not got anywhere, hence the charges. I don’t offer terms or credit so when a debt has got to 78 days overdue and the customer hasn’t replied or responded to any emails or reminders I would not consider adding interest to be excessive or harsh. However, due to a lot of Quickfiles emails ending up in junk you have to make sure you have sent them a couple of reminders yourself.
Another way of looking at this (and this question has been asked a few times on here) is to add the (some) charges on beforehand and simply offer a early settlement discount. This does need a bit of manual intervention though to make everything fully paid up if they pay early.
Personally I sit in the middle, I ensure the invoices cover overheads and admin, card/payment processing fees etc so for the most part there is some degree of interest already allowed for. The letters and reminders with fees added usually just gets the original invoice amount paid.
All depends on a) your customer base and b) how often this actually happens. Most of the time for me it’s just B2B clients not even looking at the terms and putting it in their pile for payment in their usual timely manner, EoM+30 usually. That isn’t my terms though.