Bad customer services and over billing

Worsed isp. They say you pay upfront. I was on an R150.00 deal and got billing R380.00. I sent them an email asking why was i over charged and they didn't reply. The following month on the same R150.00 deal. Their debit order returned unpaid be course they over charged me instate of R150.00 they wanted R400.01. If I could afford R400.01 then I would have upgrade to uncapped 24/7. I have sent them numerous email for the past month but no response. They have the worst customer services ever. They don't deserve any star