> Developers offering products within a game downloaded on 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 on Google Play must use Google Play In-app Billing as the method of payment, except for the following cases:
> Payment is solely for physical products
> Payment is for digital content that may be consumed outside of the app itself (e.g. songs that can be played on other music players).
That second note is critical here and the example quite literally describes the exact situation here as an exemption.
[1]: https://play.google.com/about/monetization-ads/payments/