USPS considers tubes boxes, however you cannot use tubes for some services such as Priority Mail Cubic.
When creating a label, simply select the "Box or Rigid Packaging" package type from the dropdown and enter in the dimensions as if the tube were rectangular. You will be charged the same rate as if it were a box.
If you're using a USPS-branded Priority Mail or Priority Mail Express Tube, make sure to select the corresponding service from the Service dropdown before you click the BUY button. For example, if you're using a Priority Mail Express-branded tube, you need to buy Priority Mail Express postage.
Cheapest way to Ship a Poster
USPS's Dual Ship Tubes