The triangle Priority Mail boxes- free with PM postage- work well, but only if the sections are less than 37.5" or less with clearance- although you can fit two together, if need be. I've had a problem getting them from local post offices, though- they're often out of them. Fedex and UPS have similar boxes, but iirc, they cost nearly as much money as PVC pipe.
PVC is easily precut by the store and not that expensive. And if you're getting the pipe back on the return, you'll have a nice generic (i.e., "stealth") rod case if you add end caps.
I wish I could find lighter gauge PVC pipe than what gets stocked at Home Depot, but it still isn't that expensive or heavy to mail, and the level of protection is very good.
The carpet roll suggestion is also good- it's lighter than PVC and just as strong as long as the length isn't too long, in which case reinforcing it with 3M tape helps to strengthen it.
I prefer saran wrap to bubble wrap, especially for uncased rods- lay out a length of it and roll up the sections one at a time all the way around until they're bundled and fold the ends over. Then add some sort of padding at each end, so the rod doesn't slide back and forth.
Always leave clearance at each end, and pad the ends- I once bought a very nice Bruce & Walker fiberglass rod that was shipped from the UK with the fiberglass spigot ferrule stuffed tightly against one end of the box, and it did not end well.