There are three types of courses: 1. Absolutely free courses, 2. Courses with several free lessons, 3. Courses without free lessons.
So there are two different ways to make a purchase – unblock a part of the lessons or buy the entire course.
To make a purchase you should enter a credit card details. To make a customer confident the payment is secure I put VISA and MasterCard verified logotypes, "Powered by Stripe" and "128 bit encryption" signs on the billing page.