In order to sell a single issue on your iOS app, you need to create the corresponding product ID on iTunes Connect.
Follow these instructions to create and submit all the product IDs you need:
1) Open your iTunes Connect account and click on "My apps" icon
2) Select the app you're interesting in
3) Click on "Features" and stay on the pre-selected tab "In-app purchase
4) Click on "+" button
5) Click on "Non consumable"
6) Fill in all the fields.
6.a) Reference name: This is up to you. Usually publishers just write their publication name + the date related to the issue (e.g. PublicationName - January 2015)
6.b) Product ID: You can find your product ID on Paperlit Dashboard. In the issue details, click on the "Show product IDs" button.
Just copy and paste this code on the "Product ID" field (on iTunes Connect)
7) Click on "Add language". Just copy and paste the "Reference name"
8) Take a screenshot of the billing panel of your app. It doesn't matter if it is the same image for all the product ID.
9) Click on "Save"
10) Select all the productIDs that you have uploaded and then "Submit for review"
Take in account that Apple takes 5/7 days to test and approve those.