USPS considers tubes boxes! Simply select the Box or Rigid Packaging from the dropdown when you're creating a Single Label and enter in the length x width x height as normal. You will be charged the same rate as if it were a box.
If you're using a Priority Mail or Priority Mail Express Tube, you have to select the corresponding postage on the Buy Label page. Meaning, if you're using a Priority Mail Express tube, you need to buy Priority Mail Express postage.
Totally tubular, matey!