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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. TODO: add link
Billing
If the service provider supports billing as a payment method, then they are responsible for handling the billing of the customer separately. A list of all purchased licenses can be obtained through the management API. The order ID of a license uniquely identifies each purchase. This identifier should be used to make sure the user is not billed multiple times for the same purchase.