Apple forbids this. You are required to use their IAP features and pay them their 30%.
As I've mentioned elsewhere in this thread, see the difference between the Kindle app on iOS and the Kindle app on Android. On iOS, no store. On Android, full store.
> In-store purchases: Developers charging for apps and downloads from Google Play must use Google Play’s payment system.
> In-app purchases: Developers offering products within a game downloaded from Google Play or providing access to game content must use Google Play In-app Billing as the method of payment.
> Developers offering products within another category of app downloaded from Google Play must use Google Play In-app Billing as the method of payment
The only exception they allow is if you're selling physical goods (which is the same as Apple's policy).
[1]: https://play.google.com/intl/en/about/developer-content-poli...
And they have more labor costs associated with dealing with cash. Whether that's a net win for cash or net loss depends on the particular market that the retailer is in.
Plus there's a huge gap between a 2-5% fee and a 30% fee. I imagine if Apple took a smaller cut, Google wouldn't be doing this.