Mullvad (and others) will accept cash mailed to them as payment, which is probably a better option than a prepaid card. I can't speak to the integrity of Mullvad, I personally use Private Internet Access (though they don't accept cash) due to their proven no-logs policy where they were unable to answer subpoenas because they had no info. I would not recommend a free vpn for this scenario, they're making money somehow and it's probably off of selling your activity.