USPS is the go to for my international shipping. I won't work with anyone else. Just be careful with your customs declaration, you may end up getting unscrupulous postal workers who will steal your package, or at least investigate it. Especially in these smaller nations. I used to ship many packages to a friend in Ukraine, who has since passed away. Almost every package I sent to him ended up being opened by postal workers there. Nothing was stolen, but they absolutely did look for valuables. I can't speak for Serbia, but this is my experience. Omn your customs form, do not write anything related to precious metals, bullion, coins, numismatica, etc. I used to buy dollar store picture frames, place the coins inside securely, and ship them in that, with 'picture frame' on the customs sheet. This worked, but still gave me anxiety as it is technically illegal.
On 1st March 2023 new customs declaration laws were introduced for shipments into EU countries. Being vague on the declaration form is not as easy as it used to be. Read up on it here https://www.usps.com/international/new-eu-customs-rules.htm
Get a box that looks like a product like a picture frame or the like. Pack the coins in there well with padding and seal the box along with a packing slip. Place that box inside of a second larger box with more padding. This outer box should be medium to large size. Address that box and ship it USPS.
Just remember that postel systems in other countries may not be as good as one would expect. It took 3 months for me to get a gold coin to a swiss buyer. I was just about to issue a refund or send another one. SA is another country that was fun to deal with...must admit theses 2 were the only issues I've ever had.
