The large tube (triangle) available from USPS measures (roughly) 37x4x4 which is 592 cubic inches. It is my understanding that boxes up to 1 cubic foot (1728 cubic inches) ship at the "standard" rate based solely on weight.
Sounds like your post office is a bit confused ... either that or I have been ripping off the Post Office for years.
Your right and thats why they have the standard pre made boxes. Between the system changes and the 2 new counter people they did get confused. MOst of the workers didn't even know they changed the system and barely know how to use it. The one new girl there said "I just do what the computer tells me". NOt the best attitude to have IMO. YOu should know the systems you use well enough to notice a change that big when its your job.