Definitely true. There are three that say small, medium and large. Then to provide us with free priority boxes that will weight between the 10.70 and 14.70 they have five other boxes in the same shapes that just say "flat rate".
I use those when it's too big for the 4.90 and not enough for the 10.70 and too big for the 10.70 but not enough eight for the 14.90. All the boxes do look alike. Luckily I have a small, one postmaster post office that is user-friendly and she is so helpful. Hope this adds to what KathyN posted. It's all in the print..ROFL Such is life.