I shipped some prints to Europe from Canada, but the prices and procedures should be about the same in USA. I have used round shipping tubes, rectangular boxes, and sometimes for double protection - tubes in boxes. I don't use Fedex or UPS, since they charge much more than the Postal Service.
Postal shipments have a field where you indicate the item value and whether it is a gift, commercial sample, or other. You also have to state the value.
If the value of the print is less than $100 (each country may have different limits), typically, there is no import duty. The receiver should know that limit.
Since the post office increases their prices every year, nowadays the shipping costs are quite expensive, and sometimes they may even exceed value of the actual picture, so I seldom send something anymore.