Need help for additional shipping charge

I have a decal webstore. I ship all of my decals under 12 inches long via regular large brown envelopes through US Postal First Class mail. For anything larger than 12 inches I use mailing tubes and US Postal Priority mail. Right now I am using flat rate shipping of $1.99 . The shipping cost of US Postal Priority Mail is $4.07 so I end up having to eat this cost ($2.08).

What I would like to do is to be able to add a shipping surcharge of $2.00 to any order that has a size attribute of greater than 12 inches. I thought about setting this up using weight. I would assign a weight of 10 lbs to the attributes of 12 inches and larger. Then if the total weight is greater than 10 lbs, charge $3.99 . If it is less, charge $1.99. Problem is I can't figure out how to configure the ship by weight module to do this.

Can anyone help???