I no longer have the email I originally received so I don't have the link or the license key saved anywhere. Having said that it look like you can recover your license key from Fast Spring - you just need to shoot an email to their customer support department. 

• I lost my Serial Number / License Key.
  • Please contact orders@fastspring.com with the invoice link or Order ID number.

Can someone post their activation link with the key edited out? Something like: tabsoutliner.com/key/XXXXXXX. I want to try subbing my license into the URL once Fast Spring support gets back to me.

Side note: I did some digging and it looks like Vlad (the developer of the app) was active on the app at least couple months ago. 

Same here and I've seen at least three other comments about it. Looks like something is very broken and so far I dont think anyone has heard back yet.