I looked at shipping a race bike to meet me at an event in the USA last year, and discovered what LWaB days: It's not cheap whatever you do. The customs is a killer: it would have been far cheaper for me to fly with both bikes accompanying me and then post one of them onwards once I'd landed in the US. Most airlines will allow you to fly with two bikes, so that might be the best way to go.
I know a family who have just moved to Toronto a few months ago so I can put you in touch if you want to ask about the formalities?
For anyone in the future who Googles this, the answer is as follows;
The best (cheapest, easiest, fastest) is, annoyingly, to fly with them, especially with Air Canada if possible. A flat fee of $50/£30 per bike box, regardless of how many bags you want to check, with a weight limit of 32kg/70lbs per box. I came back with 3, stuffed to the gills and it cost me $150/£85 on top of my 3 checked bags. I had to rent a van on the UK side (everyone in Canada has a mahoosive truck so getting a lift to the airport was easy) for £60.
Moving about the airport was shite because of those daft anti-terrorism barriers they have up, meaning I had to shuttle by hand each box 20m at a time, but it beats paying the >$3000 I was quoted by several shipping/courier services.