AFAIK, there used to exist a company called Borderfree which could transship goods from the US to Canada (or was it vice versa?). Looks like it was bought
. The latter seems to enable some select US stores
to transact with international customers, but this is not general-purpose, so can't be used to buy something from the US store of your choosing. At least that's my understanding. It is my vague recollection that with Borderfree you could achieve precisely that: buy from just about any US store, have it shipped to Borderfree, who then shipped it to Canada for a fee.
While researching the above, I came across this: Wishabi
. The original URL redirects to the following:DealGenius
Their Cross Border Deals link
redirects to a Wishabi page that has nothing to do with this subject though, so I don't know if they are still offering this service.
in the top-right corner seems to actually work, but they merely list certain available items from what seems to be a small selection of US stores. May be of use to someone if you find the item you want.