In-app purchases are individual purchases that are made within the app, although the app itself can be free. You can specify the price for a single issue in your iTunes Connect, Google Play, or Amazon Store account. At the same time as the price, you define a product ID there. 

The product ID is a freely selectable identifier (eg, IssueMay2017) that allows your content to be monetised through the respective app stores.

To enter the product ID Into the Manger; 

  1. Check 'Paid Content' 
  2. Respecting product IDs from each App Store 
  3. Select Save