If you are really curious about the costs to send overseas, I would advise you to go to the US Postal web site. Here is the link for pricing.
http://www.usps.com/prices/priority-...nal-prices.htm
I generally use the flat rate envelopes at first (since its smaller and the centers will fit nicely), then as it moves along, I use the Priority envelopes..then at the end I use a Priority box.....Remember you will be mailing 6 times...so you can do the math.