Send invoices to an old address following a change...

Send invoices to an old address following a change of address and charge 'administrative costs' for late payment. Customer service representative admits the error but refuses to meet the demand to drop the falsely charged administrative fee because they are "legally required to charge it".

Surpluses are said to be settled "[...] against future invoices", as indicated on the relevant invoices. Afterwards, it turns out that the credit has been deposited anyway, and that future invoice still had to be paid (with, you guessed it, 'administrative costs' as a result).

Edit: we were then contacted by customer service and the problem was resolved.