This is how I make my own boxes to ship my artwork. By making it to size you are cutting down on weight and dimensions which is great for the shipping cost
Make an Art Easel Out of a Cardboard Box
Making boxes for shipping artwork
How to pack and ship artwork — Caroline burdett
Golden State Art, 7x7x7 Inches Brown Shipping Boxes 28
Shorten a Cardboard Box. : 7 Steps (with Pictures) - Instructables
Euro record selection camp
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4 Cardboard Boxes Painting by sylvain fornaro
How to build a shipping box for paintings
Making boxes for shipping artwork
Skydives for €20.70!
Groofly.com
CanvasPak stretched canvas shipping box
Shipping Artwork Like A Professional
VShortCut - Another new development at Skydive Empuriabrava.