Bad payment allocations

I Received an outstanding account for R 270 that I settled on 28/4/2023 Received my statement on 01/05/2023 for an outstanding account of R 273.38 after I paid the account. I phoned them and query the account and have been told to send the POP to a certain e-mail for allocation. Now that we need the graduation certificate and paid R 600 for it, they declined the request as a result of the outstanding account. I think this is unacceptable and a waste of time to send e-mails back and forth to sort out the situation. I requested an updated statement after payment and up to now, didn't received anything !!!!!