For what's is worth, here's a repost of a thread on the Trains.com forum with a heads up for mail orders from the US to Canada:
I've been buying stuff mail order from the USA from years, all sorts of stuff. But today I ran into a nasty snag. I had some DCC hardware shipped express mail back on March or April. Some strange deal the US Post office now has instead of the express mail switching to Canada Post at the border as it did in the past and Canada Post collecting the duty, the US Post Office now turns express mail over to Purolator Courier! Not the I have anything against Purolator, but the custom taxes are now collected by a private company called PBB.
So 4 months or so later this PBB mails be an invoice for $21 duty and broker fees, I didn't respond for a few days until I could figure out/remember what it was for. So I mailed a check. A almost two weeks later I get a letter from PBB threatening me that if I did pay they'd send a collection agency after me and make back reports to the credit bureau to wreck my credit rating.
I confirmed with the back that they have in fact cashed the check. And called them, got voice mail, jerks like that love to hide behind voice mail, and left and somewhat hostile message, and told them to call me back if they had the nerve to.
ok, the end point I want to make is if you want to buy from the US and get fast shipping, have it sent UPS, or FedEx. But stay away from the US Postal Service and Purolator. From my past experience with UPS and Fedex that handle duty themselves. NOTE: the does not effect mail sent by "regular" US Postal Mail, just fast/express service like Global Mail Express.
I've posted this on the train forums I'm active on, if you see other places this info may save somebody problems, feel free to repost it or link to it.