white acid-free mat with foamcore backing - ships flat in a box
Pros of shipping in a tube: cheaper, less likely to get damaged during shipping, and you get to pick your own mat style and color; pros of shipping matted: you don't have to deal with matting, you only have to get a frame, and as a bonus, front of the mat is signed.
IMPORTANT NOTE ON MATTING: Since the image is 12.25"x14", it will not fit into standard 11"x14" mat opening, so a custom mat will most likely be needed. It is highly recommended that you go with option 2 - flat and matted - unless you want to cut your own mat, or you plan to have the print framed by a professional. You may be tempted to just get a 12"x14" frame and put the print inside without a mat, but that is not recommended, as there should always be a gap of air between artwork and the glazing to prevent damage from condensation, so if you want your print to last, use a mat.
