Q: Why do I have to pay again on a purchased app for using another device?

Is the iPod signed into the same ID used to purchase the app on iPad?

Is it an iPad-only app? Some developers has separate apps for iPad and iPhone/iPod touch.

If not separate apps then did yo do:

Download your past purchases - Apple Support

Contact App Support or Apple

If it is an in-app purchase then:

https://support.apple.com/en-us/HT204530

Also can yo tell us what app this is?

You may also want to check with the App developer. It may be that the require a payment for each device that you wish to upgrade to the paid version. That would be something they determine themselves. This is not an Apple app, and iTunes support may not be able to answer your question. Check with the app developer.

Have you tried:

From: About in-app purchases - Apple Support

Non-consumable (Which this is)

•Upgrade to pro edition

•Remove ads

•Full game unlock

•Unlimited hints

•Extra characters

•Extra accessories

•Bonus game levels

•City guide maps

You buy these items one time, and you can transfer them to other devices that are associated with your Apple ID. If you lose a non-consumable purchase, you might be able to download it again for free. Learn how to restore in-app purchases.

If you don’t see your in-app purchases - Apple Support