Amazon.com is not too bad, though I always order multiple things at a time to save on postage costs. Amazon.uk is probably the least cost effective (though still sometimes better than Australian shops), and never tried Amazon.ca.

The thing I like about Amazon.com is that packages arrive even faster than those sent from amazon.uk (the difference can be up to a month in transit time), and that's on regular USPS and not priority or anything. Usually I'm waiting a max of two weeks once shipment has been announced.