Because you can’t just throw anything in a plane and ship it somewhere, the Federal Aviation Administration wants to stick Amazon with a $350,000 civil penalty for allegedly shipping hazardous materials as air cargo.

The FAA said Monday that the company improperly sent a caustic chemical that leaked from its packaging, coming into contact with nine workers at a UPS facility.

According to the agency, a shipment of corrosive drain cleaner wasn’t packaged correctly, declared, and labeled. Amazon also failed to include emergency directions on the package, and didn’t train employees on how to handle hazardous materials, the FAA said.

“Amazon has a history of violating the hazardous materials regulations,” the FAA said in the release. “From February 2013 to September 2015 alone, Amazon was found to have violated the hazardous materials regulations 24 other times.”

Amazon now has 30 days to respond.

In the meantime, Amazon has been working on building its own delivery system, leasing a fleet of planes for its delivery fleet and buying a 10% stake in an air cargo partner recently.

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