Age of Origins Guide takes you into an in-depth look at most of the items you could exchange permits for.

TL:DR all the warplane items are a good deal if you buy the level 1 event pack. Everything else is more expensive.

Permits do expire so use them before the event ends. You will be able to see them in your item bag additional time after the event ends and you can click on them there to go spend before they become expired chests.

This event happens it seems each month with a face lifted theme.