You could choose from a few options: send the print as is flat using cardboard or something similar to keep it from getting damaged; use a cardboard tube and roll the piece into the tube; Mat the print and send it along with cardboard, etc. for protection.
Personally I like to mat and ship. If the customer does not like the mat they can always replace it - I think the mat gives it some extra protection. Just make sure to sign the print and mat in case they swap it out.
Shipping rolled in a tube is also a good option. However, depending on the paper you use it can be problematic due to creases created during the rolling process.
Martin Bailey has a good video cast on using a tube for shipping that I would recommend checking out -
Cheers,
Fred