It basically sounds like they are purchasing an elevated user level.
I think for this you should use both eCart and Security Assist.
First you should go through the Security assist User Level Authentication tutorial on the security assist support page, in the solution recipes section:
to become familiar with security assist and user levels.
On registration, you could default them to the free level, then give them the option to purchase an elevated level.
On checkout, you could update the userlevel column in the database to the level they purchase.