There shouldn't be anything that would make any real impact on your 10GB a month bandwidth - for absolutely ages (three years) I too had a 10GB limit and I don't do a hole load of music/video downloads so I never even came close to the 10GB limit (which has now been upgraded free to 40GB).
I would have to ask where you are getting your bandwidth figures from as there can be a huge deal of confusion over what it responsible for downloaded data ?
As a resident antivirus the web shield will be scanning all downloaded data - this will show up as avastSvc.exe - the main avast service controlling the avast shields and scanning, etc.
But avast for the most part isn't responsible for the initiation of what is being downloaded - that is a browser request for data, be that a web page display of a download, etc. So instead of being assigned to your browser process avast redirects the call through its proxy so it can scan the data before being displayed in your browser.