Flat versus tube is a trade-off, there's really no right answer. Flat shipping can get very expensive as the print sizes get larger, and even sandwiched between protective layers the flat print is probably more likely to get damaged en route (the risk goes down the better you pack it, but costs go up). The tube is safer and cheaper, but the customer has to deal with curl, especially when dealing with some of the stiffer art papers (Epson Exhibition Fiber is so stiff that Epson wasn't even willing to sell it in rolls initially).
If you want to see the sort of elaborate packaging needed to really keep a flat print safe, try ordering a mat or frame from Frame Destination. Their packaging is excellent, and I've never had a mat arrive with any damage from them. Some of the other online companies such as Frames By Mail don't package as well and the mats can arrive with dinged corners or other damage.