Save money when you sell online.
Anyone selling online knows how expensive new boxes are. The US Postal service offers free shipping material with you use their Priority Mail service, but there are some items that are so large that they won’t fit into the Postal Service boxes. If you are shipping to a business then you may get a better rate using FedEx or UPS. You are going to need a plain box to ship in.
Don’t buy them! It is really easy to find cardboard boxes in good shape in all sizes.
Go back to school. Really. Your local school goes through LOTS of boxes on a daily basis. Boxes that contained packaged food, boxes that contained paper or boxes, boxes from cleaning products - lots and lots of boxes. Ask the principal and find out who is in charge of recycling the boxes. You will most likely find someone who is more than happy to give you some boxes instead of having to break them down. If you collect boxes on regular basis you may even get your favorite sizes stored for you to pick up.
I have not purchased a box in years. I get to save money and recycle.