They do have a vested interest in your happiness, even if they do not offer a guarantee. Two situations that might cause you to run into the problem of zoning differences between the U.S. and Canada is when you are researching information from texts that happen to have been published in the other country, or when you first import plants. In general, American zones differ from ours by one zone- usually what they will term Zone 5, we will term Zone 6. Try to be sure that you have at least one good reference text that refers to plant hardiness in your own terms. Now, we've talked about macroenvironment , the big climate picture, but there are ways of trying to bring thru plants that are warmer by nature than the rest of the area, or your garden.
Most gardens have hot and cold spots.
These microenvironments are often a zone or more warmer or colder than the prevailing zone, and so may be able to support special plants that you really would like to try. Full sun areas that face south, especially those close to walls, are usually warm. If you have pets, you can get to know your own garden's microsystems pretty soon. Cats know microsystems. On a sunny cool spring day, they will always find the warmest spot.