Extremely Over billed

I took out a 3G promo through Mobiletalk, Telkom billed me for 20G. Mobiletalk then gave me the roundabout trying to sort the **** out. 6 Weeks later i get a bill of R2619 of which they say R1419 is for migrating from 20G to 3G, Firstly i was incorrectly billed for both 20G and 3G and secondly i didn't downgrade as I initially took out 3G.