The staples Canada website doesn't seem to have any corrugated boxes smaller then 12" deep-( Maybe it's an in store item only.
OTOH they have some padded envelopes. I think I've got an empty 11x14 paper package. The kind that held 10 sheets of paper. How about putting the prints in the paper package. Then stuffing that into the padded envelope? Should even be enough room for some cardboard in those big envelopes.