My first swap (France), the post office did not ask what was in the package (a small bubble envelope). The second swap (France), they asked, and I said "collectibles", she looked at me funny and handed me a customs slip to fill out. I wrote "hobby supplies" on the form. No problem there, either. The third swap (Lithuania) went the same way (but that was the most recent and I have not received confirmation that it was recieved).
I don't want to put "US Coins" on the sticker for obvious reasons, and I take steps to hide the contents. They also ask the value of the contents, and I put the real value of the coins.
Lately, I've been reading that vague descriptions, such as "collectibles" or "hobby supplies", might give cause for customs inspectors to open the package.
Any advice on good mailing practices from the more experienced traders?