PayPerAccess

If you need to grant access to your website only to those peaople that had paid
a fee or have done something you need before they access: this plugin allow you
to setup an “access URL” that permit you to access all website contents.

If users don’t access through this URL, they’ll be redirected to another page
(for eg. a payment page) and then redirected back by that page to the “access URL”
if they pay or do something you want.