Service Fees for Prepaid electricity

I would love to know what this service fee is that FNB charges when purchasing prepaid electricity. Just bought R150 worth and only got 18 units!! R106 towards this so called "service fee". This is the first time I've seen it. Thought it was a mistake and purchased a further R20 worth. Service fees of R15 and only 2.5 units of electricity! FNB please explain yourself!