you wont need to change anything if you change the charge to use wight instead of total weight.
the cart calculates the totalweight as wight * quantity.
when you put one item in the cart, the total weigh calculation returns 1 (1 * 1 - 1)
when you add a second item to the cart, the totalweight no longer equals 1 (1 * 2 = 2) so the shipping is not longer applied.
you need to use the weight column in your calculations not total; weight.