The accepted answer is correct, but we should take into consideration another aspect about purchasing via smartphone/tablet. This recalls @JaranJhanjee's downvoted answer which is wrong but contains a little important truth

The 30% fee is charged by Google for in-app purchases according to their terms

• Paid and Free Apps

  • App purchases: Developers charging for apps and downloads from Google Play must do so by using Google Play's payment system.
  • 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).

Google always charges the 30% via its in-app billing API. But there are cases in which such an API should be avoided to not incur in an excessive charging

• If navigator maps are usable across more devices (especially non-Android devices like iPhones o MS Phones)
• If maps will be delivered packaged in an SD card at the customer's home address

In such cases, the seller is allowed to implement a different payment gateway like Google Wallet API or PayPal

For apps there are is only one possibility to start a subscription:

If Google Play Store asks you before you download an app if you want to purchase an app for a certain amount of money, then this is an one-time fee. To my knowledge apps can only have a one-time-fee or they are free (to be downloaded).

Repeatable fees are are usually billed as in-app-purchase. That means you open the app and at some point the app can ask you if you want to start a subscription. That will open a Google Play dialog containing the price, when the subscription starts and so on.

in-app-purchase can also be a one-time payment. But that is then visible in the Google Play payment dialog.

Active subscriptions can be viewed in Google Play Store app in section Payments and Subscriptions. The same is possible in the web interface which also has a section Payment and Subscriptions