In-app purchases: Developers offering virtual goods or currencies
within a game downloaded from Google Play must use Google Play's
in-app billing service as the method of payment. Developers offering
additional content, services or functionality within another category
of app downloaded from Google Play must use Google Play's in-app
billing service as the method of payment, except:
where payment is primarily for physical goods or services (e.g., buying movie tickets,
or buying a publication where the price also includes a hard copy
subscription);
or where payment is for digital content or goods that
may be consumed outside of the app itself (e.g., buying songs that can
be played on other music players).
Developers must not mislead users about the apps they are selling nor
about any in-app services, goods, content or functionality they are
selling. If your product description on Google Play refers to in-app
features to which a specific or additional charge applies, your
description must clearly notify users that payment is required to
access those features.
Source: https://play.google.com/about/developer-content-policy.html