Elizabeth H. from ColorfulClay says
First Class international is the most cost-effective way to ship internationally and can be used for packages up to 13 ounces.
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13 oz. is the limit for domestic U.S. First Class. International First Class goes to 4 lbs.
International First Class is definitely the way to go, as long as your package is 4 lbs. or less. (It must also meet some dimensional requirements, but that's seldom a problem.) Make sure you get an accurate weight, because every ounce increases the cost significantly.
I figure only two shipping amounts for International. One is Canada. The other is Australia, which also works for almost everywhere else. (I don't ship to "everwhere else," but to certain specificed countries, based on what my third party insurer (ShipSurance) will cover because I think that's the safest way to go.)
The buyer pays any customs charges, duties or tariffs, so you don't need to worry about that. But make sure to make a note of that in your policies, and it's also good to remind the buyer in the order confirmation email. (And don't ever let a buyer talk you into marking the item as a gift, because that's illegal.)
Mailing label and customs form are easy if you print labels through Etsy. The customs form is incorporated right into the mailing label, and all you have to do is sign it.
Lastly, make sure to insure any shipment that you can't afford to lose or replace. Use a third party insurer, not usps. If you use the Etsy mailing label, you can opt for their insurance, which I think is U-Pick. I insure separately through ShipSurance because I've heard good things about them and because I can insure for the entire amount, price + shipping, so that I'm fully covered.
I hope that helps. It's really quite easy once you do it a couple of times.