If you tap the icon for courses that have not been purchased in the World Tour, a button will be displayed that leads you to the purchase screen.
(For iOS)
The purchase will proceed using the payment method (credit card, iTunes card) associated with your Apple Account. Please contact Apple Inc. if anything is unclear regarding the purchasing procedure.
(For Android)
The purchase will proceed using Google Play payment methods from your Google Account (credit card, payment along with your telecommunication carrier bill, Google Play gift card). The availability of payment methods may vary by country.
Please contact Google Play Help Center if anything is unclear regarding the purchasing procedure.