I have built crates for vises before. And I also bought a Wilton 3-1/2 inch that had just been shoved into a box. Luckily the post office taped it up after it punched its way out through the corner of that box.
I've also used USPS Priority Mail flat rate cardboard boxes for vises. I cut pieces of 1/8 inch plywood to put into the bottoms and sides to strengthen the cardboard.