Whenever your customers cannot pay in due time, you should ensure they’re aware of the consequences. Under normal conditions, if you buy from somewhere services or goods and don’t pay up, it’s considered taking a loan. I say “normal conditions” since it’s assumed you haven’t bargained for special treatment, i.e. longer payment terms for an example. 

So, what happens if you take a loan? You’re expected to pay for the usage of the money. You’re using essentially someone else’s money for an extended period and in this case for a period not initially even intended by the moneylender.

This something that you’re expected to pay for the usage is called “interest”. Just the same as loans are earning interest to the lender, so should do overdue receivables.

The best way to enforce interest on overdue receivables is letting the customer know of it by adding this information already on the invoice itself. It can be in the form of a simple sentence saying that overdue balances exceeding the payment deadline shall be applied an interest of a determined %. Note here that this interest is actually per day and not per annum, so using like 5% isn’t all that sensible. Normally the rates are somewhere around 0.5-0.7% per day. It’s not expected to be cheap since as I mentioned before, it wasn’t the initial assumption of the seller that they won’t get their money. They’re “forced” into lending and thus the interest, if you’d apply this for the whole year, would yield into a very expensive loan.  Maybe consider taking an overdraft to pay up? An overdraft from a bank is a lot cheaper.