Envelopes & Boxes
I called this file 'building with matchboxes' as it includes templates not only for the match boxes but also some to help make it easier to construct with them.
Phone charging/door hanger tote
Pop Up Box Templates & photographic step by step instructions
It folds flat to slip easily into a 14/14.5cm square envelope - so long as you don't go too crazy with the embellishments! Though the one I made as the example would need a slightly taller envelope, as the teddy on the back panel sits above the top edge of the card!