You want to offer a certification course - the complete content, as well as the quiz(zes) at no charge, but you do not want to enable the user to get their certificate unless they purchase it.
For this scenario, you will need to know a bit about using the Site Designer, Other Products, and the Course Page Manager.
Step 1: Create a usergroup named PAID USERS.
Step 2: Build your course, along with all content and quiz(zes).
Step 3: Somewhere in the course, create a SECTION that only displays to users that are in the usergroup PAID USERS. Inside the section create a subsection - and put a course certificate in there. This section will only display to users in the PAID USERS usergroup.
- Edit the section. From this page, select "Add a New Requirement". When the prompt appears, select "Usergroup" is PAID USERS, and save your changes.
Step 4: In the shopping cart, create an Other Product - this will be the 'certificate' purchase. In the Product Triggers section put a check next to "Add the user to" and then select the PAID USERS usergroup from the dropdown.
Now, when a user purchases the product, they will be added to the PAID USERS usergroup, which will cause the section with the certificate to be displayed.
An alternate way to do the certificate (if you need something more customized compared to the generic course certificate) is to use the certificate in the quiz. Make sure you EDIT the quiz, and remove the [[[CERTIFICATE]]] placeholder from any email or response page template for that quiz. Next, in the SECTION you have set up to show up for users in the PAID USERS group, instead of adding the course certificate, add the REPORT CARD EXTENDED item. This will give users (in the PAID USERS group) access to the quiz certificate.