Nancy says "If you can fit your items into the USPS flat rate boxes that seems to be the best deal. If you do it online you pay and fill in the customs form then and they just drop it off at the post office. It's pretty easy and you can look up the prices online and figure those out ahead of time."
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In my experience, usually the flat rate boxes are not a good deal, though sometimes they are, and it depends on weight.
Canada: For the small flat rate box to be cheaper, the package has to weigh over three pounds. For the medium flat rate box, it has to weigh more than six pounds. For the large flat rate box to be cheaper, it has to weigh over eight pounds.
Other most other countries: For the small flat rate box to be cheaper, the package has to weigh two pounds or more. For the medium flat rate box, it has to weigh more than four pounds. For the large flat rate box to be cheaper, it has to weigh over eight pounds.
I made a chart for my bulletin board that shows me how much international packages cost vs. the flat rate boxes so I don't end up paying more than I need to. (Let it also be duly noted that the domestic flat rate boxes are rarely a good deal.) In case anyone is interested, here is my frugal shipping tutorial:
http://www.facebook.com/notes/lauras-last-ditch-vintage-kitchenwares-small-appliances/frugal-shippin...