System overload

I been a long standing customer of ABSA and their was always par excellence but I made a mistake of paying my bond earlier (the 16th instead of the 30th) thinking I must be saving somewhere but I was wrong. Now I'm getting phone calls to say that I am 1 month in arrears but I've been paying my monthly installments. I have notified the call centre agent to change my date of payment to the 15th but the irony of doing that is that i will have to pay an extra R300 on my monthly installments to my bond is fully repaid. All this because their "system" cannot calculate early payments.