Mike Caldwell spent years turning digital currency into physical coins. That may sound like a paradox. But it’s true. He takes bitcoins — the world’s most popular digital currency — and then he mints them here in the physical world. If you added up all the bitcoins Caldwell has minted on behalf of his customers, they would be worth about $82 million.

Basically, these physical bitcoins are novelty items. But by moving the digital currency into the physical realm, he also prevents hackers from stealing the stuff via an online attack. Or at least he did. His run as the premiere bitcoin minter may be at an end. Caldwell has been put on notice by the feds.

Just before Thanksgiving, he says, he received a letter from the Financial Crimes Enforcement Network, or FINCEN, the arm of the Treasury Department that dictates how the nation’s anti-money-laundering and financial crime regulations are interpreted. According to FINCEN, Caldwell needs to rethink his business. “They considered my activity to be money transmitting,” Caldwell says. And if you want to transmit money, you must first jump through a lot of state and federal regulatory hoops Caldwell hasn’t jumped through.

Because the process is so complicated, Caldwell has stopped taking orders for his popular Casascius bitcoins — which have become one of the most recognizable images of the thoroughly intangible digital currency. In recent months, the feds have cracked down on many other bitcoin operations in similar ways, including Mt. Gox, the most prominent online bitcoin exchange. But Caldwell’s case is a little different. He doesn’t think he transmits money.

Caldwell doesn’t accept U.S. dollars or any type of fiat currency. You send him bitcoins via the internet, and he sends you back metal coins via the U.S. Postal Service. To spend bitcoins, you need a secret digital key — a string of numbers and letters — and when Caldwell makes the coins, he hides this key behind a tamper-resistant strip.

So long as you can keep your Casascius bitcoins safe, nobody can learn the key. To date, Caldwell has minted nearly 90,000 bitcoins in various denominations. That’s worth about $82 million at today’s exchange rate.

Caldwell takes a fee of about $50 on each coin he mints, but he argues that sending the coins through the mail is not a way of transmitting money. He thinks the coins should be viewed as collectibles.