Why not just use the built in subscription feature.
In group permissions just set the default users group to only allow access to a forum that talks about paying and leading to that url.
You can even set all the forums as teaser forums , where they can see they exist but do not have access to the topics unless they subscribe .
Then you create a new group that is what they subscribe to has the permissions you want.
So when paid they will automatically be granted full access with no human intervention.
In fact you could even tier several subscriptions so each group has more permissions than the prior.(bronze,silver,gold)
Not sure what ubb version you are on.
