Create usergroup, create paid subscription
via admincp/paid subscription, add usergroup and forums to that subscription and set forum permissions.
You won't really need a shopping cart you can just put a upgrade button in the NavBar link it to the payment screen.
You can also make forum viewable and when members click on the forum it will redirect they to the payment screen, see this link.
https://vborg.vbsupport.ru/showthrea...87#post2453187