Unpaid item charge

on the 14th September i paid R1000 into my account . swiped at baby boom Tokai for R330. the payment for baby boom was only processed the 22nd of September and at the time i had little or no money in my account which meant it went it a minus . Fnb then decided to charge me R110 for this . i am failing to understand why i got charged when initially when i made the payment on the 14th with my bank card i actually had money in my account and have the incontact email to prove it . i called the call center and the agent decides to tell me i had no money in my account when i made the purchase for R330 ,how the hell can i make a purchase with no money in my acc . i had money its just that the payment was only processed 8 days later which again is not my fault so why the hell am i being charged R110 for this . i am not standing for this and want that charge reversed