How much does it weigh? I usually ship First Class International if the package is less than 4 pounds. I also check whether I'm better off using a flat rate Priority Mail box, which includes insurance.
Check the USPS site for any prohibited items (here it is for Scotland/UK)
http://pe.usps.com/text/imm/fh_014.htm#ep3576772If you don't have the buyer's phone number, you may want to ask for it as the customs form has a space for it. (If I don't have it, I just put my phone number.)
You can easily ship through Etsy just like a domestic order. The "shipping label" will be 4 pages. Be sure to sign and date all 4 pages. Take all 4 pages to the Post Office, where they will stamp it and affix it to the box in a plastic envelope. Good luck!