I have been trading with Japanese pin collectors almost since my first internet trade.
Pack your pin in the same protective manner that you would for a domestic trade.
You can mail your pin 1st class in a standard bubble envelope and it will probably cost about $1.75, and take 7-14 days to get there.
If you want to get it there faster, you can put your protected pins into a
small global priority flat rate envelope for $5.00, and it will get there in 3-5 days. (There is also a large flat rate envelope, 8.5 x 11 inches, which costs $9.00. I've never needed one this large for a pin trade. I've sent as many as 8 to 10 pins in the small envelope.) The envelopes are available, free of charge, at any post office.
Insurance for either method is extra and starts at $1.10 for up to $50.
For either method, you need a customs form, USPS form 2976, available at any post office.
For convenience, I keep some of these envelopes and customs form at home with my packing supplies.
My own preferences:
I try to stick to multi-pin trades with Japanese partners -- three or more pins at a time.
I then feel it is more than worth it to send the pins in the global priority flat rate envelope.
My theory? I think the foreign postal services recognize our special class envelopes and treat them a little more carefully. (Our global priority and global express envelopes are incredibly similar.) I have no proof, but it makes me
feel that my package is more likely to get where it's going.
