I'm Canadian (I'm sorry) and as most conventions happen state-side, I have the pleasure of learning from my mistakes with customs. If you are shipping across the border, may I suggest Fed-Ex. They have their own customs brokerage and will charge you later for any duties (heh heh- dooty

). I haven't been charged for sending books, except for a $25 brokerage fee but I have received a huge bill once for shipping t-shirts across the border. Maybe pack those in your suitcase.
I was advised by a customs officer that some airport terminals are not equipped to charge duty (heh heh) as they are not designated "ports". So declaring goods to be sold in your suitcase may just end up with those items being confiscated. Recommendation, fly through a designated port airport or ship it by courier. I am in no way endorsing smuggling your t-shirts.
I still have a bit to learn about paying the duty (heh heh) for bringing goods to the states: Is there a way to be refunded for any of the goods that I didn't sell?
Does anyone have any experience with cross-border relations?