You're probably going to have to go a step further; something like Sift Science or MaxMind MinFraud along with automated SMS/voice verification of a unique phone number per trial. There isn't a carder in the world that wouldn't love to use a VPN service that offers free trials. They want the VPN to use as a proxy when testing stolen credit card numbers on other ecommerce sites... and they have plenty of cards to use to sign up for free trials with you first.
This is definitely true and something I've experienced running a proxy service with 500+ clients. Fraud percentages are WAY too high right now. We're solving it by simply checking the billing address vs. IP geolocation. Hopefully that helps.
