iTunes makes backups of all app data, including Shopper, whenever you sync. You can use those backups to restore all your app data, including Shopper, to any iPhone or iPod that you have authorized with your iTunes account. So if you ever have to restore your device or buy a new one, you will be able to transfer your Shopper data. Just make sure you regularly sync with iTunes. That's when backups occur.
Here are a couple of tech support articles from Apple about backups:
Shopper Support Team.