How can a server determine the amount of an iphone in-app purchase ?

The server receives the purchase's receipt sent by the app and uses this to validate it.

This returns some informations like a transaction_id, product_id, etc; but no amount or user details.

Since the amount depends on the currency and the user's country, how can I get the amount of the purchase on the server ?

iphone ios in-app-purchase itunes
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