For the fourth time in as many months, U.S. aviation safety regulators have proposed a fine on Amazon.com Inc. for allegedly shipping hazardous materials by aircraft.

According to the Federal Aviation Administration, in August 2015 FedEx Corp. workers at a sorting facility in Cary, Ill., discovered a leaking package that held two 14-ounce bottles of a flammable, ethanol-based hair tonic. The shipment, which was flown from Ruskin, Fla., to Algonquin, Ill., wasn’t packaged or marked properly to show it contained hazardous material, the FAA alleges, and shipping papers didn’t provide required details, including emergency response information.

The agency proposed a $78,000 civil penalty against Amazon for the incident, adding to three other fines of $350,000, $78,000 and $52,000 that regulators proposed in June for similar violations. Regulators in the U.K. have also charged Amazon this year with similar violations, including an attempt to ship lithium–ion batteries on passenger aircraft that are barred from carrying the batteries.
 

A spokeswoman for Amazon said in a statement that the online retailer has “developed sophisticated technologies to detect potential shipping hazards and use any defects as an opportunity for continuous improvement,” adding, “We will continue to partner with the FAA in this area.”

The enforcement actions come as Amazon is adding logistics services and building operations, including a leased fleet of cargo airplanes, so that it can handle its goods in its own network. Insiders say the retailer has long-term ambitions to haul and deliver packages for other retailers and consumers in competition with FedEx and United Parcel Service Inc.

Brandon Fried, executive director of the Airforwarders Association, a group representing freight shipping agents, said the penalties show the complicated task the company faces in training workers and gearing up operations to handle so many products on its own in a tightly-regulated air shipping business.

“once the FAA is on your back, you’ve got to make sure you have your house in order because they’re not going to relent,” Mr. Fried said. With Amazon, he said, “There’s a lapse there—attribute it to their growing too fast and not surrounding themselves with the right people at the right time.”

Write to Erica E. Phillips at erica.phillips@wsj.com