Have you entered into a contract with them for the supply of goods or services that govern all transactions etc? This contract may say that if you don’t pay by a certain time then they can charge interest, get a debt collector involved etc and that they will pass on these collection costs to you. If you haven’t signed a contract for the on-going supply of goods or services, then these terms may, for example, be printed on the back of their invoice or provided at the time of sale.
If you’re unsure I’d ask them for further info i.e. point to where it says you are liable to pay interest / collection costs.
Practically it may be a case of saying “sorry I’ve paid this late” – if late payment is a one-off or rare then they might waive some or all of it if you have a good relationship with them but they might take a different view if they have already chased payment etc.
I hope that helps