hmmm, the more I think about it the better it looks
If they pay a nominal fee to get access = proved themself that they have a CC and want to use it. Same as AVS.
Since the traffic is already pre-approved, it's of higher quality.
Submitters will realise that you have higher quality traffic = higher quality submits. <--- good reason why people would sign up and retain. Of course there's more things you can provide that make them sign up and retain, but that's one thing.