You get hit with big fees called "Brokerage" if you use any courier company. (UPS, FedEx, etc.) USPS (United States Postal Service) is a government agency, so to speak, and has no such fees. So, USPS is always cheaper than going with a courier, and in my experience, not much slower at all. Sometimes using a courier can't be avoided due to the size of the items, or a seller who only uses a certain company.
Duty and GST laws I can't help you with, since I don't know the exact laws, but it seems hit-and-miss with everything I have bought. I never know what customs is going to decide to charge me. One tip is to make sure the seller puts the actual selling price on the package. I've had sellers jack up the value thinking they were doing me a favour for insurance or something, but this only caused me to pay more tax.