In App Purchases
If the integrated 3rd party vendor has the ability to process receipts then Backstage can take the receipts from the front-end applications and pass them along via the record receipt API
The 3rd party is responsible for processing the receipt and communicating with the respective app stores to ensure the payment has gone through in order to store the entitlement.
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The 3rd party integrations supporting this functionality currently are: Cleeng, Generic Integration