Is this app compatible with WP8.1?
- Yes it is. Every addon will work also with WP8.1

Must I pay again for the addons?
- No, every addon must be paid only once. After an update, every addon is still paid.

In the last days a lot of phones got the update to Windows Phone 8.1. Unfortunately after the update, every paid app and item has to be synchronized again from the store and this need a Wi-Fi connection and some hours.

So, please be patient and don`t pay the addon again, your addons will be there after some hours. If it still doesn`t work:
- be sure to open the store ==> so that a login to your account is made.
- reboot your phone if after 2-3 hours they are still not there ==> this will force a new synchronisation.
- use a WiFi connection ==> Only in this way, apps and app settings are restored.

Even if it is not directly releated to this problem, this link could help you in the future:
http://www.wpcentral.com/restoring-app-purchases-windows-phone-reset