You can set that some services are available only for customers who purchased a prepaid package.
In your Bookeo Appointments account, go to "SETTINGS", click on "Services offered and additional options"
Click on the desired service, and go to the "Price" tab
In the "Package only" section, select that customers can make a booking only after purchasing a prepaid package.
When customers click on that service, they will be asked to sign in the customer area and buy/use their prepaid package