You can set that some services are available only for customers who purchased a prepaid package.
1. Go to Settings>Services offered and additional options
2. Click on the specific service, and go to the Price tab
3. In the Package only section, select Customers can make a booking only after purchasing a prepaid package.
How can customers book services that require the purchase of prepaid credits?
When customers visit your booking page and click on that service, they will be asked to sign in the customer area and buy/use their prepaid package