Advtech charging their account errors on the client!

Advtech in correctly allocated my children's school fees payment now I am being charged a fee for that mistake, which is not mine after paying for the whole year's school fees up front! This is the second time such a thing is happening and I feel like it's a way of for the business to siphon money out of me. I tried calling the account department and they say they cannot tell where the error is, they promised to send me an email that is binding that they will resolve the error to no avail meaning, I have no proof of accountability from them.