given that you're in the Thunderbolt forum, i assume you're using a Thunderbolt--but i frequent other forums too since i like to read up on all types of issues in Android.
are you a Thunderbolt user or using a different phone?
since the hotspot feature is free until the 15th, i would assume that the wired option is going to have leniency and not be charged until the 15th as well. however, knowing Verizon, i wouldn't be surprised if they were to charge extra.
i'm uncertain how much/what the rate of charge is (per MB). an easy way to find out is calling a CSR and asking to check on your bill to see if there has been any unusual activity or extra charges added. if they say no, then you're good to go.
but i wouldn't tether, via USB, after the 15th--just to be safe.