@deadlydecibel, use $3.49 as your base shipping cost and $1.50 as the items shipping cost.
So now when someone orders one item it will be $1.50+$3.49 = $4.99
but when they order 2 it will be 2*$1.5 + $3.49 = $6.49
You can see that this way every additional item is charged $1.5 extra.