Two alleged dark web drug dealers sold almost $9 million worth of fake Adderall online, only to see their illicit empire fall apart by buying postage on Stamps.com.

Over the past six months, according to court filings obtained by Quartz, undercover federal agents purchased Adderall, a prescription ADHD medication, on Dream Market, a now-defunct online emporium specializing in narcotics and stolen personal data. The pills came from a seller identified in charging documents as a Chinese-born US citizen named Tzu Yang “Issac” Lin. The drugs turned out to be counterfeit, containing only methamphetamine. They were supplied by a Taiwanese national named Meng Ting “Leo” Hu, and mailed to buyers using the US Postal Service.

The two men, who live in Southern California, have been operating since at least 2017. Prosecutors say Lin was responsible for “more than 28,000 individual sales,” worth in excess of $8.8 million, on Dream Market alone. Customers paid for their drugs in Bitcoin.

The dealers appeared to know how to cover their tracks online. Users can only access dark web marketplaces through special software that ensures anonymity, and cryptocurrencies like Bitcoin are meant to be untraceable. But Hu made a crucial mistake that exposed their identities, leading directly to their arrests.

“A review of the postage found on all three of the undercover purchases shows that they are pre-printed postage purchased through Endicia, a subsidiary of Stamps.com,” the government’s criminal complaint says. “Analysis of the information associated with the Stamps.com account used to ship the parcels containing methamphetamine led Agents to identify [Hu] as the true account holder of the Stamps.com account.”

Now that they knew his real name and address, the feds put Hu under surveillance. They saw him “transferring boxes” to Lin in his garage on multiple occasions, and later tailed Lin to a post office in City of Industry, California, where they watched him deposit 87 Priority Mail envelopes into a mailbox.

A judge approved a search warrant allowing the agents to retrieve the parcels.

“A search of the eighty-seven (87) parcels revealed approximately 1.573 kg of tablets in total, peach in color, oval in shape, and bearing pill imprint ‘b 974′ on one side and ’30’ and a half tablet score on the reverse,” according to an affidavit from the Food and Drug Administration’s Office of Criminal Investigations. “These pills are consistent with the tablets previously purchased by undercover agents which laboratory testing revealed contained methamphetamine.”

The next day agents executed simultaneous search warrants at Lin and Hu’s homes.

At Lin’s they found a “large quantity” of pills identical to the ones seized the day before. Lin admitted to the scheme, and told officers that Hu was his supplier.

Hu was less cooperative. He ran out the back door when agents showed up. Officers finally arrested him a few doors down. Inside Hu’s place, agents found a large number of pills, a pill press, various “punches and dies,” and “a large quantity of powder.”

Hu confessed to taking part in the operation with Lin, and told agents they caught him while he was in the middle of pressing a new batch of pills for Lin to sell.

Declared a flight risk, the court ordered Hu detained pending trial. Lin is free on bond. Authorities confiscated his US passport to ensure he doesn’t leave the country.

Lin’s attorney, Louis Shapiro, declined to provide further details or a statement on the case. An attorney for Hu, Mona Soohoo, did not respond to a request for comment.