It's a Peel-off sticker storage box and was only £2.29 from Hobbycraft.
Previously my sets were stored with my other unmounted Clarity stamps, on A4 laminated sheets in a RUB. This works well for the other stamps but I was always worried about losing one of the small letters. I know Barbara very kindly says that if you send an SAE to Clarity she will replace any lost letters but you can guarantee that you won't notice a missing one until you need to use it!
I cut two pieces of thin card to 9.5 x 22.5cm (I allowed a bit of room at the end so that they can easily be taken out of the box) and stamped all of the stamps on them (I know I've stamped an x upside down but I wasn't going to start again!).
Then I laminated them and trimmed them down leaving a .5cm border of laminate.
Like with my large boxes, I cut a piece of baking parchment to put in between each layer so that the stamps don't stick to the laminate above.
When using them you can have one layer in the base and one in the lid. If any do fall off they won't go anywhere. Plus, as I've stamped out the letters it will be easy to see if any are missing before I put them away. Hopefully even I won't be able to lose one!!!