It's great you already have a Branded Mobile Application. In case you've decided to set up in-app purchases, you'll be using a unique Product ID for each single issue you intend to sell.
You firstly need to set up in-app purchases for single issues on your Apple and Google Consoles.
Warning: It is absolutely necessary that the Product ID in your App Store Connect coincide with that of your Joomag account.
If you need to set up product ID for a single issue follow these steps:
1. From My publications page open the publication where the issue is located.
2. Click the Edit Setting button.
3. Copy and Paste iOS and Android Product IDs in the respective fields.
4. Double-check if the product ID corresponds with that of your consoles, and only when you're sure it does, click Save and Close.