I package my original drawings between two boards, as shown below, and send them using Royal Mail's Track and Trace service. Prints are sent by Royal Mail Standard First Class and are rolled up in a tube, the ends well-sealed with masking tape.
My latest piece on Artfinder.
How I ship my artwork
Here's a drawing I did today based on one of K.C. Wu's beautiful photos. (See his Google Plus page: https://plus.google.com/+KcWugonggi/posts.) I will take a better picture tomorrow during the daytime, as the lighting wasn't very good for this one.