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There is no way to get the transaction ID if you are using Windows.Services.Store namespace. If you are using the old Windows.ApplicationModel.Store namespace, you could get the transaction ID in the PurchaseResults object. In the old in-app purchase process using Windows.ApplicationModel.Store namespace, the transaction ID is used to check the product fulfillment for consumable in-app products-Enable consumable in-app product purchases.
If you are using Windows.Services.Store namespace, there is a different way to check if a user had purchased a project-Get info for add-ons for the current app that the user has purchased. You could check the purchase using the Store API directly without sending it to your own server. Of course, if you need, you could send the purchased product information as you want.
Thank you.
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