First of all, it is technically illegal for the USPS to ship coins to many countries. The complete list of postal regulations can be found here:
http://pe.usps.com/text/imm/immctry.htm
If you're sending to Canada, UK, France, or Germany, you will be fine. For anywhere else, I would look it up first. Some countries require import approval from the central bank (such as Czech Republic), or require that the coins only be sent in insured packages (such as Russia). Some nations prohibit bullion metals but allow base metal coins. Others only ban the mailing of their own coinage (such as Austria).
Every outbound package needs a completed CN-22 customs declaration form. I live in a neighborhood with a large foreign expat community that frequently sends packages back home, so the CN-22 forms are freely available at the counter. Your community might be different, so the CN-22 forms might be kept behind the counter. Ask a clerk.
Also, you cannot send international shipments from a self-help kiosk. You must send your shipment with a clerk at the cashier's window during business hours, since they have some questions about the contents that they are legally required to ask you in person. ("Does this package contain flammable or perishable materials...")
On the customs form, I always describe the contents as "numismatic supplies". (If you use the word "coins", you are asking for trouble.) I once used "hobby supplies" on the form, but the postal clerk demanded a clearer description, and I used "old bus tokens" instead.
I estimate a 10-cent average value per coin for valuation purposes. If I'm shipping American coins overseas then I use the actual USD value. Keep in mind that a high dollar value of a package's contents will raise a red flag and be further bait to corrupt postal workers who might be tempted to swipe it, so lowball your value estimates.
I usually package my coins in bubble poly mail envelopes that I buy from my office store. I wrap 5-10 coins in a piece of kleenex so that it forms a flat padded strip, then I tape the strips together onto a piece of cardboard with several layers of clear packing tape. No metal is visible, nothing will rattle, but the package stays lightweight and rigid.
USPS charges $12.75 to send a small package anywhere via standard shipping. Sending a package registered can cost an additional $20. Insurance can cost a lot more, and there is a minimum $625 value threshold before your package is even eligible. Heavier packages, like my recent 9-lb box to France, cost me $60. I can very easily understand why most American collectors refuse to ship beyond Canada due to the high expense of USPS fees.
It usually takes a few weeks for my packages to reach their European destinations. Have patience and trust. I have had more issues with packages being tampered with inside the USA than with overseas postal services. (I once lost a package of Swiss coins that was sent to me from another American. That package arrived to me, but had been cut open and all contents removed.) If you have any trouble, there is an official complaint form that you can file with the USPS. Departments that receive a high level of complaints get audited by the Postmaster General, which can help ferret out corrupt employees.
Good luck, and happy swapping!