Zain e-Go: Show me MY Usage before you charge me!

I have this Zain e-Go device (Huawei E220) and on 3 occasions I had to pay more than the subscription fees and got my bandwidth capped at 16 kB/s because I went over the 30 GB/month limit.

What I don't understand is since Zain knows how much I'm downloading, why doesn't it show me the bandwidth usage per day?

I don't mind paying when exceeding, but Zain/MTC has a history of mis-billing and now I can't tell why I'm paying extra, unlike the case with calls & SMS.

I'm not the only device user, so it doesn't help to install a program on my own machine and log the traffic.