Create 2 shipping rules:
1) Base Rate
Triggers: Total Number of Items > 0
Calculation:
Flat Rate of 2
2) Incremental Rate:
trigger: Subtotal For Any Column:
if the subtotal of column [Quantity] is [>] a value of [1]
Calculation: based on Multiple of column subtotal
Subtotal of column [Quantity] [times] [0.50] [minus] [0.50]