Looks like Tumblr changed and broke some stuff again. It’s been fixed now. Please wait a couple of hours, or update One-Click Postage manually from the XKit Control Panel.

To update manually:

1. Click on the X icon on your dashboard
2. Click on One-Click Postage (or One-Click Postage Classic) on the left bar
3. Click on Re-install / Manually Update This Extension
4. Refresh the page

(Update: One-Click Postage Classic is also fixed.)