TomTop offers the Tomshoo 30-Litre Ultra Lightweight Foldable Backpack in Blue, Green, or Rose for $7.69 plus $2.65 for shipping. (Kokorest via Amazon charges the same.) That's $4 off and the lowest total price we could find. It folds into a zippered pocket.

Note: The countdown timer on the product page does not reflect an actual expiration time and resets daily.