Deducting more than what is said on the statement

I recently financed a vehicle and after going through my contract I saw that they are going to deduct my payment on the 26th of each month although I sometimes only receives my salary on the 28th of each month. I requested that my payment/deduction date to be moved forward to the 1st of each month. They've send me an amendment statement stating my new premium amount but deducted more than what was showed on the amendment statement. After querying, they've said it is service fees for the movement of my payment date, but nowhere do they state that the deduction is going to be more than the payment amount on the amendment statement so that you can budget for it.