FNB service fees/ unpaid even though it already went off your available balance

This is the second month that ive picked up FNB charges me for debits card transactions that went off my available balance and lying in my "pending basket" then all of the sudden i am charged R125 for those fees that already went off my avalable balance and then all off the sudden when the company clear the funds FNB charges me R125 again because i dont have available balance at that stage this is ridiculous !!! Needless to say i need to waste my airtime to resolve this....