Exorbitant bank charges

Fnb is robbing the customers.we dont normally look at the fees they charge. When you swipe your card, the purchase is deducted immediately from your account. Should you have R500 in your account and you purchase for R500. The R500 is deducted immediately and your account balance is R0. Then when the company you purchased from banks their money after a few days or a week, FNB charges you a fee again as there is no money in your account regardless of the fact that the money was already deducted at the time of purchase. This does not make sense at all. That means if i purchase for R500 on the 1st of feb. I need to make sure i still have another R500 in my account til the 5th of feb. If i dont they charge you a fee of more than R270. They charged me a fee of R270 for a cwrd swipe of R330 and a R90 fee for swipe of R25.

Please can this bank be investigated as this is not good customer etiquette

Thank You