you would use eCart providing the subscription as an item to purchases and checkout.
You would want to implement user level access on the site, and have the user registered and logged in at a basic level before checkout.
On checkout, when the transaction goes through, you could update the user level to an advanced level that will be given access to the premium content.
See the tutorials for user level on the security assist support page and adding registration to the checkout process on the eCart support page.