I buy via the Amazon website, which I have bookmarked on iDevices, or more often, on my Kindle Fires or my Android phablet, which offers the Kindle app with the store baked in. I don't know whether Google charges Amazon a cut, but apparently it's a workable deal for both and makes for convenience for customers with Android devices.
I'll never buy books from iTunes, because they can be read only on iDevices. You buy from iTunes, you're stuck accessing your books from iDevices, or walking away from your books if you switch to non-Apple devices. With Kindle, your books go where you decide.