You know, Heinlein did something like this once.

Basically there was a door that led to like two bedrooms and a bathroom. The actual stuff on the other side of the door existed statically in a location in a universe, while the door itself was inside a universe travelling pickup truck of sorts.

Using that idea for the charms, one could posit that when you expand the interior of, say, a trunk, the space is static and exists somewhere. Were you to destroy the trunk or cancel the magics, that connection to the expanded space would be 'cut off' and whatever was stored in it would be lost.

Which is kind of interested.


Alternatively, it could function like a Bag of Holding when turned inside out. Namely, that it expells EVERYTHING contained within at high velocity.