Your account is connected to your Apple ID/iCloud and syncs automatically.
To use Invoice Maker on another device, make sure you’re logged in with the same Apple ID/iCloud as before.
Then, open the app and go to Settings > Business Information > Switch Profile (at the bottom). This will display all profiles stored in iCloud.
If you need to reactivate your plan, go to Settings > Restore In-App Purchase.
Give it a try, and let us know if you need any help!