You can go to usps.com and go to "Track a package" and then put the number in....the number normally starts with an "LC" and ends with a "US". If you did not keep the number off the green slip, you can look at your receipt from the PO and it should be on there as well.
I have read on other posts that you can also go to the Canada Post website and put in the number to track it further into Canada.
Here is the link the Canada Post:
Canada Post - track a parcel
Here is the link from the discussions on the ebay store boards about this very subject:
Ebay Discussion Boards - How to Track package to Canada
I found that link to be most interesting....hope this helps and good luck!
Marcia