Debit order on wrong date, was charged no funds

I have a contract with TELKOM on the contract I specifically told that and wrote it on the form, which I had signed in the store that I should be billed on the 20th since that's when I get paid when I check my statement for June there's a HONFEE of R90.00... which was debited from my account since Telkom debited my account 19th and I had no funds in my account , I don't understand why they debited on the 19th since that was not part of the agreement, as I person you make plans on what you will do with the funds in your account when you debited R90.00 which is a huge amount for something which I had no clue about, which was not part of the agreement due to when they charged me on the 19th I had no fees in my account, that's why I ask them to debit on the 20th, I mean why don't they think of their client all they do is debit, I was about to use my card it kept on saying my funds are less but I knew how much I had , if I didn't check my statement I wasn't going to know about Telkom debiting me on the wrong date.