FNB Electricity Service Fee

Nowhere online I can find how the Service Fees work when buying Electricity via the FNB app. I have bought Electricity for ZAR 350 a few days ago and had to pay ZAR 25,44 service fee. Then I bought ZAR 150 worth of Electricity and had to pay ZAR 50 service fee. That is 33%! This must be an error.