There may be different rules for long guns vs. handguns. However, non FFL holders in the US can't use the mail but must use a commercial common carrier to ship handguns to a US FFL holder. I'll leave it up to others who may be country specific. You would also have the EU Rules to contend with. Bear in mind that packages are often x-rayed. Here are two pistol shaped bootjacks I bought out of France. Note the packaging as it was when I received them. If I were you, I'd check with a firearms importer in Slovenia. I assume US made Colts and Glocks find their way to Slovenia above board. Gil
[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]