I previously bought Dropbox Sync, but can't restore the purchase. How to enable it?

If you are running the same version of pwSafe where you originally bought it and you are using the same Apple-ID as you originally used to purchased the add-on, you can tap "Buy" again and you won't be charged.

If you are using pwSafe 2, but originally bought the add-on with pwSafe 1, then you need to re-install pwSafe one from the app store, restore the purchase with the old app and then perform the upgrade again using pwSafe 2's settings menu.

pwSafe 1 is not listed on the App Store anymore, so you'll need to go to the "purchased" section, which is accessible on your account, by tapping your picture.