Buying Parts from Canada

I've shipped quite a few parts to the states through the post office and had no reports of problems with extra charges.I've heard that some states will collect sales tax on new parts but I've never heard back from anyone I've sold parts to saying that this happened to them.I usually write that the parts I'm shipping are used when I fill out the form at the post office so maybe that helps...also,declaring the parts as a gift could be a way around that if it's a problem in your state.