If it were as simple as you stated I would do as you suggest. But this phone would have to go back to Costco. I would get my $200 refund for it. The $350 ETF is for the Verizon contract service. I then would have to start a month to month plan and pay $30/mo for a new phone.
Why would the phone have to go back to Costco? Why not just keep it? You paid for it. You are paying the $350 ETF for cancelling the contract. Why would you have to return the phone?