AnyBoxToday.com - great boxes!
Usually I don't respond to emails, but I got one from AnyBoxToday.com and I checked out their website and they looked legit, so I took them up on their offer of free samples.
They have GREAT boxes that will fit the "slot of doom" - that size restriction that Canada Post uses to determine if you can ship by lettermail or parcel.
AnyBoxToday.com and look for them on facebook too.
Give them a try!