Standard bank VAF

Standard bank VAF is the worst. I told them which day of the month I get paid on and asked that they deduct on the following day, and they decided to ignore that and chose a date they liked. I called even before they were to make the first deduction , and I was told I can change the deduction date after the first deduction has went off. I did that, and all I got told was that it has been changed, come the deduction date I found that the installment has now increased, which I wasn't told that it would when I change the deduction date. All their consultant could tell me when I called is that this is the first time that they see an increase in installment because of a change on deduction date, and that maybe I should change it back, because its probably the dates I changed to bluh...bluh...bluh, did that it increased more. I called again, now they said, no changing the deduction date doesn't increase the installment, its probably the interest rate , I told them the interest rate on this loan is fixed, now they had nothing to say. This doesn't make sense to me, that if I change the deduction date that the installment increases, and it's so wrong of you to not tell me that and just increase it. And by the way my first vehicle was financed by Absa and I could change my deduction date with no increaments on the installment, so really I don't understand what's going on with Standard Bank.