For starters, on the eCart support page:
See the following tutorials:
Applying Register/Login to your checkout
Automatically populating checkout fields
Then in the archived documentation section of the security assist support page:
see the "User Level Authentication" tutorial.
With a paid membership site, the work flow will be:
* User Registers and logs in - When the registration is created, the user is assigned to a low level user group, using the user level authentication recipe, you can create user levels for basic users and paid members.
* Using eCart, you create a method for the user to purchase a paid membership.
* Once the checkout is complete, the User level is updated to paid members level, this then allows them to access the paid for content of your site.
On check out, you can store an expiration date in the users table, just like the userlevel, this can be stored in a session at login. You can create your access rules to compared the expiration date against todays date.