Thats what I've been using but Im out
i see you're still new to this
https://store.usps.com/store/browse/subcategory.jsp?categoryId=subcatMSS_B_Free
select the size you require, then you can order up to 25 of each
it ships to your house for free. i usually ship priority when i sell on ebay so it works out, it's a flat rate fee.
sometimes though, if you ship by weight it comes out to less than what the priority flat rate is

so i just invert the box and hide all the priority lettering on it and ship it. sometimes a shoe box will be too long/wide for these flat rate boxes, but you can do some mcgyver sht to them and still make a nice lil enclosure for shoe boxes using these free boxes. these are my back up when my nike/fnl/champs etc boxes run out
TL;DR: FREE BOXES FROM USPS.