There is an AddOn available to handle the payment and registration of the LearnDash.
WooCommerce AddOn for Learndash
This add-on allows you to sell your LearnDash created courses with the WooCommerce shopping cart.
Easily map courses: When creating a product in WooCommerce, simply select the courses you want to associate to the product.
Associate one, or multiple courses: Supports one or multiple courses being mapped to a product which allows you to create course bundles.
Works with any payment gateway: Choose any payment gateway offered by WooCommerce and your customers will be enrolled into the course after successful payment.
Works with WooCommerce Subscriptions: Charge customers a recurring fee for course access.
Automatic course access removal: Removes users from courses when refunded or canceling a subscription.