You can get 18" x 24" mailers with adjustable height from U-Line for around $4 each. I mark them up and add it to the shipping cost. Bigger shipments: bigger boxes. I used to order by the case.
If you are only shipping once in a while, shipping costs can be a game stopper. For me to drive across town to pick up a box, assemble everything, then drive to the shipping place and pay for shipping is going to cost the client well over $100.00. Big online companies have stacks of boxes @$2 each and packing stations with all materials handy including scales and a computer to print out labels. The shipper comes to them. The same box that cost me over $100.00 to assemble and ship costs them about $15-20.
Many online companies are now offering "free" shipping. They bake the cost into their listed price. I'm good, and know a lot of the tricks, but I simply cannot compete with them on any level except for design. So I offer my clients design services, then order the signs, then either have them drop shipped or the client can pick them up if he wishes. Either way, the client gets the benefit of our design expertise and experience with vendors in addition to paying half what he would have paid if he would have bought these from a local sign shop that prints on-site. The benefit for me is higher profits, lower overhead, and increased sales because nobody in town can match me for quality or price.
Note: Well they could match me if they wanted to. I know several shops who have retired their printers and outsource these days, but their overhead is still higher than mine, and they haven't gotten around to hiring really competent designers yet. They will in time, however, so I'm just taking advantage of the current situation and maxing out profits now!
https://www.uline.com/BL_1304/Jumbo-Kraft-Easy-Fold-Mailers