Support buying IAPs using `https://buy.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy`

Description

It seems to take a guid GET parameter (e.g. https://p29-buy.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy?guid=bd6dacc17e08f495dfd21e72a66453e9d0fd3af2) but apart from that I have no idea how this endpoint works.