Tons of people I know used Uship.com. It is like an Ebay for shipping so you post what you need and you get bids from tons of companies and you can read the feedback on all of them.
In my case (in Canada as well) I just decided to drive it back thanks to the long Easter weekend. Hell of a trip, from Santa Monica to Ottawa. 4,800km on the car!
As I had new tires, new PVC window, new MAF, new oil, etc I was confident I would make it.
At the border, took me 30 seconds (no kidding) on the US side and 10 minutes on the Canadian one (as they need to fill out a form).
As mentioned, make sure you fax what they ask to the US Customs 72 hours in advance (basically Certificate of Title, Bill of Sale and a cover letter with all the details on what you are doing - exporting the car with VIN# ABCD).
On the Canadian side, yes, they did ask me proof of payment but as they do NOT mention that anywhere on the CBSA/RIV/CRA website I told them it was NOT my fault I did not have it and they let me go, no questions asked.
If you will ask the seller to put a lower amount on the bill of sale to save on the taxes, make sure you pay him using two different wire transfers/paypal/whatever so at the border you show them the one that matches the agreed price on the Bill of Sale.
This way they cannot refute at all what you are saying you paid for, assuming it is indeed in the range shown on the Blue Book for example.
Then you will pay 11.1% tax at the border plus $100 for the air conditioning. Once registering the vehicle you will have to pay 8% PST in Ontario at the MTO.
The day I crossed, the CAD was higher than the USD so I ended up paying almost nothing on GST!
Any questions feel free to ask as I am currently in the middle of getting the car plated in ON.
CR