I got seriously caught out that way early on and as a result will no longer deal with "commercial" type dealers in the USA. Emails asking for explanations were ignored completely - never again!!! Private individuals are a pleasure to deal with though.
It's traditional to complain about your own postal service, everybody does, but I know from experience that Royal Mail and Australia Post would take a lot of beating. US post can be quite slow if you're dealing with someone near the far coast, and as for Canada - it's quicker to go there yourself.
GG