What's the cheapest method of shipping [parts] to/from the United States/Canada?

The members of Canada DSM have the following advice:

Bob Brown: " USPS is about 50% more expensive , but you just have to pay GST."

Jerry Hong: " I've found it to be least expensive to use a local trucking company that does freight to any US state. UPS/Fed-Ex wanted $1300US to ship a transmission/transfer case/diffs/4-bolt rear end from Halifax to Houston, Texas. The local trucking co. is doing it for $375US."

"Total_Eclipse" : " All i can say is, use Canadain Freight. They have a sister company in the USA called Consolidated Freight. They are awsome for shipping big parts."

Also see the information regarding brokerage fees, above. Parts may be subject to GST in all cases.