Penalized for paying

So I decided to pay my account earlier via eft. But then I got charged a penalty fee!

I emailed them trying to find out how and why I got charged after paying my account, just for them to tell me that if you don't pay your account via eft 10 days before the debit date you get charged a penalty fee.

So because I decided to pay a few days earlier I got penalized. What the actual! How can this even be legal. No where does it say that it has to be 10 days before!! I moved to telkom thinking it would be better. I am no regretting that big time! Worst cellphone service provider ever!!!!