Teather is not designed to prevent charges, your 1 and only problem is believing that you can teather to your PC and use BIS.
What you are trying to achieve is impossible, the teather app your using only works with BIS in USA as they have actual apn's assigned to their differently CAPPED BIS packages so they do not give 2 ****s weather or not you are using your limited data on your phone or your PC.