You are correct. The internet pass through is kind of like reverse tethering. Which carrier are you with? You could always call customer service and find out what they say.
I doubt there would be any reason to charge you for anything as you are not using anyone's towers and/or data plan. You are using your existing internet to get the connection (much like using your home wifi to do the same). I don't have this particular phone, so I can't say that for sure, it is just a semi-educated guess. I will make sure someone who does have it can try to give you more help though.
Do you not have a wireless router? You could always use the wifi instead if you do. If not, do you know how to set up the internet pass through? I read it as you know how to do it but are just worried you might get charged for it. Correct me if I am wrong.