Brad Dowdy reviews the Lihit Lab Teffa Bag In Bag over at The Pen Addict.
In one instance it was a standard stationery bag, containing various pens and paper to use and share. In another instance, it was specifically built as a portable letter writing kit, with loose paper, envelopes, postcards, stamps, washi tape, etc. Seeing both of these as stand-alone bags as well as slotting into other messenger bags and backpacks piqued my interest.
I've always viewed the Lihit Lab Bag in Bag products as standalone cases. They make a lot more sense to me now as their name implies: a bag to organize things, like a letter-writing kit, inside another bag.