FNB Prepaid Electricity - Fluctuating 'Service Fee' Charges

I love FNB, but their prepared electricity service is cooked.

23 April 2021, I bought electricity for R350, got 95.6 units and got charged R115.92 service fee. 5 May 2021, I purchased R350 electricity. Received 124.4 units and paid a R60.48 service fee. 23 May 2021, I purchased the same amount, R350 and received 179 units and paid a R5.04 service charge.

That’s 3 transactions in one month, with the same value R350, units received, ranging from 95kWh - 179 kWh and service fees ranging from R5.00 - R115.00.

I then looked at other prepaid electricity transactions I did within that same month, because I realized I’ve been purchasing electricity quite frequently as of late to the same meter number.

21 May 2021, I purchased R100 worth of prepaid electricity, received 10.1 units and paid a R80.64 service fee.

None of this makes sense. Fix it!!!!!