The one case where I know someone was charged, they charged it to the cell phone account holder - so if the account is in someone else's name, that is probably who would be charged. I am not sure if they contact you prior to charging or returning the device you sent in...maybe someone else will chime in...
If you are charged, there is no getting the phone back or having the charge reversed without upper management approval. You will be sent photos of problem and a description of why you were charged, the Verizon manager can see this information as well.