How do I combine StoreKit with Promo Codes for In-App Purchases?

Question

My app uses StoreKit to handle in-app purchases. Now I would like to make use of the recently introduced promotional codes feature. The documentation explains how to generate and redeem codes. However, I have no clue in which way a redeemed promo code would be reflected inside my app (somehow, the user should be notified about the product that (s)he gets for free).

Currently, the app generates SKProductRequests and receives SKProductReponses to get the prices, titles and further information for each product:

- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response;
{
    NSNumber* price1 = [response.products objectAtIndex:0].price;
    ...
}

Now I would either expect the price for an already redeemed product to be 0 which is currently not the case. Alternatively, I would suppose that there is some API which tells me whether a user actually needs to pay for a single instance of a product.