The issue goes like this:
1)I'm selling an info product on my main website using custom payment gateway
2)After the payment confirmation I redirect them to create a profile in the members site using the Membership plugin, by creating a free Subscription Plan.
The problem is that a user that buy the info product, and redirect to the Registration page, he can share the URL and by that help others to bypass the payment step.
How that problem can be fixed?