If you're talking about buying from my site (EPO), we send it in a padded envelope along with the appropriate customs documentation attached and included. The Postal service going from Canada to the US usually isn't too bad, but what we found was that even with the appropriate documents it really came down to the luck of the draw as to whether the postal service choose to "inspect the package for customs" or not; in general with small packets they check about 1 in 4, so if yours is the one then the customs will add about a week of processing time to cross the border.
All in all it's a bit hit and miss, as some areas seem to get the deliveries quite quickly, while others take much longer, so we say "Up to 6 weeks" just so that you're aware of how long it CAN take. From the feedback I've gotten I would say the average time for the economy service is about 2-5 weeks, and the average time for the regular postal is about 1-3 weeks.

Thanks,
-Rob