Prevent curl: Carriers charge the greater of calculated dimensional weight or actual weight. A flat box is dimensionally larger than a tube and costs more to manufacture.
The flat box fee does not include shipping. It covers extra packaging so your posters arrive without curl. One box can hold up to about 30 posters.
Less than nine posters are usually rolled and shipped within reinforced tubes. The number of posters that may be rolled depends on tube diameter, original poster thickness, and whether lamination is applied, maxing out at 6 to 9.