Some people have found a new way to turn fool’s gold into the genuine article.

In an effort to popularize its new one-dollar coins, the United States Mint has offered to mail coin orders to buyers free of shipping charges. Enterprising “travel hackers” quickly figured out that they could buy the coins, rack up frequent-flier points on their credit cards, then deposit the coins to their bank accounts to pay off their credit card bills. Officials began catching on when they noticed repeat orders adding up to as much as $600,000 worth of coins; they got another clue when banks reported receiving deposits of coins still in their Mint wrappers.

“We’ve used them to go on trips around the world,” Jane Liaw told NPR, saying that she and her husband are planning trips to Greece and Turkey, “all on miles and points.”

But it’s not illegal.