Lates fee on overdue invoice

Usually is requires some agreement at the outset of the business transaction.

It depends--did the invoice itself, work order, sales contract, or other paperwork warn the customer that you had the right to charge interest on overdue invoices? If it did, you may; if not, you can't, since you can't retroactively change the terms under which your customer or client bought goods or services from you. (Of course, going forward you can include this term on your paperwork and charge a late fee or interest.)