wrong in arrears account

Im getting tired of your poor service and accounts processes. i received an email saying that am owing you R500 and i must pay it within 2days... what R500? the one you over charged me for Router? last time i checked you fixed that amount and i paid what was credited to me. you even listened to the initial call between me and your consultant when she was offering me data and told her to cap my data and the issue was solve last month but today you sent me such email. you need to fix your account processes. I was looking forward to upgrade one of my contracts but with this poor service i cant. Solve this