As others have mentioned. Use the calculated shipping, or use flat rate shipping supplies. Go to the USPS website and see which supplies would work best for you and order some, they are free to use, you just have to purchase the appropriate shipping when you use them. Not all of the items on the USPS site are flat rate so that is where the calculated shipping would come in.
I would think a Padded flat rate envelope might be what you need, since they are 12 1/2" long when closed, but I don't know the how easily the plate could be bent or otherwise damaged. If the padded envelope won't work for you, I would offer a regular priority mail shipping cost, for instance if you select one of these packages from USPS 1092, 1095, or 1097 boxes, and probably a medium flat rate box (the one that opens on the ends not the top), that way if one is less expensive for the distance, the customer can choose which one.
At any rate, go to the USPS site and click on "send" then on " stamps and supplies" so you can select a package that will work for you. That way you can put shipping method, size of package and weight into the listing and avoid the need to mess around after the fact.
