I ship 24 x 60 quite a bit. In the US, I have a local UPS store that helps me a lot.
I will totally wrap up the print in wax coated Kraft paper, the waxed side facing the print. I first build up a series of sides that are just bit higher than the print with the protection. Then I tape these down. I then cover the face with foam core 3/16 and the back with card board. On of print of this size I will have 2 strips of support built into the frame so these add protection also in shipping. The sides being a bit higher I put the foam core on this and them tape it all up. You have built a box around the print by doing this.
Then I wrap it in large bubble wrap, and take it to the UPS store I use. They will build a custom box that fits the print and allows 3 inches of foam peanuts.
Net is cost a lot do this, but the prints I have shipped get there fine.
Simply canvas, does a similar thing i.e. a box around the print, inside another box, but their shipping costs are much lower due to volume.
The key is to protect the face of the print and keep something from punching through from the back where it's open. On the the back I tend to 2x the cardboard to to make sure.
I get the card board from a local frame shop that throws it out every week. All Conservation Clear glass ships with 2 excellent pieces of cardboard and this is what I try to get, if not that the sides of a 40 x 60 foam care box, which will also work.
Paul