Bravopilot did not recognize the box
Let me see if I can explain this properly.....
Those triangular tubes do have 4 sides/dimensions.
Looking at the end of the tube, you see 3 flat 'sides'.
The 4th dimension is the flat end of the tube.......measure from the flat bottom to the ^ point at the top.....in a straight vertical line.
Typically, the 3 flat sides are the same. So, when asked for the dimensions....measure length + 1 side + the vertical.
And you are good to go!!
BTW...this not 'The Rex" way....it is the USPS way.😅