Bill more than agreed on

Took out a new contract with a 500 MB on a tablet for R 89-00 for 24 months , first debit went off and R 127 was deducted. I phoned in last month and they told me their was an extra service charge that wont be deducted this month, I've received my invoice this month and R127 was deducted again. On my account they have billed a laptop, modem exc. for a total of R127.22 and not the agreed advertised R 89-00 contract !!