Ill gladly pick up any used boxes you have received from online shipments and reuse them for shipping.
I use this for shipping shrimp, that way I can just use the flat rate boxes from USPS.
https://www.homedepot.com/p/Greenfiber-25-lbs-Cellulose-Blown-In-Insulation-or-Spray-Applied-Insulation-INSSANC/311574454?MERCH=REC-_-searchViewed-_-NA-_-311574454-_-N&