Changing Shipping Costs depending on Quantity Ordered

I need to have shipping costs change depending on how large the order is. For instance, if someone orders 1-3 items shipping is x, if they order 4-6 items shipping is y, and if they order 7+ items shipping is z.

Is there a way to achieve this in the plugin? If not, can someone give me some pointers on how I might go about adding such a feature myself?

Thanks!