Your Store’s Purchase method is called when the user opts to make a purchase. Your store should take the user through the checkout process and call either the OnPurchaseSucceeded or OnPurchaseFailed method of the IStoreCallback.
Your store should supply a receipt and unique transaction ID; if the application has not already processed a purchase with the supplied tranasaction ID, Unity IAPSee IAP More info
See in Glossary will invoke the application’s ProcessPurchase method.
When the application acknowledges that a transaction has been processed, or if the transaction has already been processed, Unity IAP invokes your store’s FinishTransaction method.
Stores should use FinishTransaction to perform any housekeeping following a purchase, such as closing transactions or consuming consumable products.