Hi,
you can do this way:
you put for ex. 5.00 for 1 yard for USA, then you add 1.50 for every other item
When I sold 2 yards for an australian customer, i put:
10.00 for the first yard + 5.00 for the second = 15.00 that was exactly the amount of shipping cost for the 2 yards
Convo me if you want more explanations!