Did you ever think of that? That a round mailing tube could roll off the truck? Probably not. Seems obvious though, doesn't it?
"Dammit, Jim! I'm an ARTIST, not a LOGISTICS EXPERT!"
The reason I completely switched over to shipping via UPS was an experience I had with regular U.S. Mail one Christmas. I had a pencil portrait that needed to get to Ireland in time for Christmas. I had sent it in a round mailing tube (mistakes help us learn!)
I heard from the customer that his artwork arrived very late on Christmas Eve, but it all ended well since it got there, but it was a real nail biter. UPS gives me better tracking. I am happy to pay a bit more money for less stress and to be able to give better customer service.
I still have to mail larger artworks in a mailing tube, but now I often put the round tube inside a triangular-shaped one.
"I'll put that box inside of another box, and then I'll mail that box to myself, and when it arrives... I'll smash it with a hammer! It's brilliant, brilliant, brilliant, I tell you! Genius, I say!"
Most of the portraits I do are 11"x14", so I mail them flat. If you have ever tried to flatten out anything that has been rolled up for some time, you know how difficult it is to handle.
That's why I prefer to send flat whenever possible. It's easier for the customer.
And it doesn't roll off a truck.