See /wiki/spaces/MAPI/pages/49676828
Purchasing
Endpoints overview
...
A user may purchase a License for themselves by calling the create license endpoint. In order to create a license, some method of payment must be provided. At the moment only one method of payment is supported: "billing". With this payment method, no further payment details are necessary, but the user is charged through the service provider's billing system.
Purchase restrictions
Some users may have restrictions set on their account that prevent them from making purchases. These users are marked with the user flag "LICENSE_PURCHASE_RESTRICTED". See User flags for more information.
Billing
...