It isn't illegal. Coinabul made it up as a reason for why they didn't send his gold. Maybe they pocketed the insurance payout from the other poster as well?

Coins; banknotes; currency notes, including paper money; securities of any kind payable to bearer; traveler’s checks; platinum, gold, and silver; precious stones; jewelry; watches; and other valuable articles are prohibited in Express Mail International shipments to the Czech Republic.

General Import Restrictions
The following items are not acceptable for carriage to any international destinations unless otherwise indicated.
...
Money (coins, cash, currency, paper money and negotiable instruments equivalent to cash such as endorsed stocks, bonds and cash letters).