If it is only a couple of T-Shirts then I would suggest Expresspost (by Canada Post), they sell pre-made envelopes that you can stuff as full as you can, it gets there in 2-3 days guaranteed, and is cheap.
You can also buy the envelopes and then send at your nearest postbox.
Regular mail only takes 4-5 days as long as it is a somewhat big American city that you are sending it to.
My experience in sending stuff to the states, is that you avoid fedex UPS unless it is really valuable and/or time sensitive.
i've shipped lots of personal goods from canada to the states. the best way is to use post instead of courier. i suggest you check out the us customs website for information specific to your needs as the rules vary.
that said, anything made in north america is not subject to any duties (nafta).