I have been collecting for 25 years and never bought a "bike box" to ship a bike. Why would you?
Every local bike shop is very happy to give them away for free ..... local cardboard recycling cost them money, so giving them away is money in their pocket. Just ASK.
P.S. On a side not, if you have never spent any money at your local bike shop, and you are in the bike hobby, than it's totally fair if they charge you for a bike box. Support your local bike shop ..... not Walmart!