Hi again aogunade,
Based on the guides I've read USPS is generally the cheapest way to ship your items overseas. There may be other options which you can find by searching on the internet, however it would be a bit of a challenge vouching for these companies like - Link hidden: Login to view and Link hidden: Login to view since we have not really tried using them.
You might find the tips in this guide helpful - Link hidden: Login to view
As for suppliers for refurbished laptop suppliers, please checkout the following posts -
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Hope this helps :)