You need regional rate boxes George, they come in diff sizes. And cost less to send than medium and large flat rate boxes. Go to usps.com and order them, they are free. While you are doing your order also order the other priority mail box sizes you need, and pick up some padded flat rate envelopes, they always come in handy, not just for mailing stuff. If you mail long items pick up some of the priority mail tubes.

I shipped a netbook in a padded flat rate envelope. There is also a legal sized flat rate envelope that might be what you need depending on the screen size.