Only on your iPhone. Kasovo has no server and no accounts. See the privacy policy.
Settings → Backup → Export gives you a ZIP file. On the new phone: Settings → Restore and pick the file. Free for everyone.
In the EU the legal guarantee for new goods is at least 2 years from delivery, regardless of the store's or manufacturer's commercial warranty. Kasovo shows both separately.
Check iPhone Settings → Notifications → Kasovo → Allowed. Reminders are scheduled locally and refreshed every time you open the app.
Unlimited items and PDF claim export. One-time purchase, no subscription. To restore: Settings → Restore Purchases.
Refunds are handled by Apple: reportaproblem.apple.com.