A Rock Hill, South Carolina, woman received a package of oxycodone pills in the mail by mistake Saturday. The recipient was expecting to find a yoga mat she ordered from Walmart, but she found more than 20,000 pills of illegal oxycodone instead.

The pills were reportedly meant to be mailed to the woman's vacant condominium, but the mailman knew she had moved. The street name was misspelled on the label, but the mailman thought it was a mistake, The Herald reported Monday.

The package was later rerouted to the woman's new address. The recipient of the pills, who remains unidentified, reportedly called the police moments after her discovery. The oxycodone pills' street value amounted out to $400,000, Cox Media affiliate WSOC reported Monday. The package contained two large bags with roughly 10,000 pills inside each, with each individual pill valued at $20. 

The Rock Hill Police Department (RHPD) claim the pills are being qualified as mostly counterfeit. The oxycodone, which was mailed from California, was likely pressed from powder into pills in Mexico. 

"Vacant apartments, homes where people are gone to work for the day and someone's sitting outside tracking the package, waiting for the trucks to make the delivery," Marvin Brown, a drug unit commander for the RHPD, told WSOC Monday. "Most of this is counterfeit and it comes from other countries and it’s being shipped in."

Local law enforcement is investigating where the package was originally intended to be delivered.

Search and seizure is a right under the Fourth Amendment that protects the examination of letters and packages. Sending drugs through the mail, however, is a federal offense. However, felony charges will only be brought against violators if there is considerable evidence that proves the individual is guilty of the action.

South Carolina law details what makes the practice punishable under its Code of Law.

"A person must not be convicted of a criminal offense under this section unless it is shown by clear and convincing evidence that the drug, pharmaceutical preparation, chemical, chemical compound, or device would not have been obtained but for the fraud, deceit, misrepresentation, subterfuge, forgery, alteration, falsification, concealment, or other prohibited act allegedly practiced by the accused," South Carolina's Code of Law states. 

Postal inspectors can request to obtain a search warrant if they suspect that the contents of the package may prove to be in violation of federal law. 

The U.S. mail system is a common method for drugs, especially marijuana, to be delivered in small quantities, according to a January report from the San Diego Union-Tribune. Despite the legalization recreational marijuana in states like California, the delivery of drugs by mail remains illegal. 

"[Drug dealers] are sending drugs through the domestic postal service in packages that are not required to provide the same electronic data that they would have to provide if they sent them through a private express carrier like FedEx, UPS and DHL," Tom Ridge, a former Homeland Security secretary and Pennsylvania governor, told Fox Business in April. "China is the major culprit. It’s open season there. Just send it through your postal service and chances are it will get through without detection."