Options for Center Mounting
Unfortunately we don't offer centering mounting, but to accomplish the same look of a center mount you can put a border around the image and order the print and mount at the size required. See How to Order an Unlisted/Custom Size Print
For example, to emulate an 8x10 center mounted on a 10x12, use a photo editing program to place your 8x10 image on a 10x12 canvas. The 1" border all around the image can be any color you like. Save that file and order it as a 10x12 print and select your preferred mounting service.