How can I set different shipping rates for t-shirts and sweatshirts?

[SteelToeBrew]

Hi. We sell both: t-shirts, and sweatshirts, and would like to (need to) have two different prices for each one's shipping. We have some boxes from the local post office, so: the one that is $8.05 will hold a t-shirt, and the one that is $13.60 will hold a sweatshirt. What we want to be able to do, is if people want to order TWO t-shirts, that they would be charged (2) t-shirt shipping rates (so, $8.05 + $8.05), or one sweatshirt box, so $13.60 (because we can fit two t-shirts in 1 sweatshirt box). We tried giving everything a "weight", and managing the shipping profiles that way, but it's not seeming to work when we go through all the steps on our website. Help! 

Thanks.