Over the past several years, Amazon has dramatically expanded its fleet of aircraft to help it ship Amazon orders to customers faster. The growth has been so dramatic that it has fueled speculation Amazon would eventually begin flying cargo from other companies, not just Amazon packages, on its jets, whose tail fins are emblazoned with its smiley-face logo.

But Amazon has, in fact, already begun quietly transporting cargo on its planes for at least one prominent client: the U.S. Postal Service. The arrangement could portend bigger ambitions on Amazon’s part to compete with the likes of UPS and FedEx in the shipping of goods by air.

One person who until recently worked at Amazon’s air cargo hub in Baltimore told The Information that they regularly loaded and unloaded USPS cargo from Amazon planes. And in a piece of information buried in a 2019 filing with securities regulators, Sun Country Airlines—one of the air cargo companies Amazon works with—says it is obligated to carry cargo from the postal service on behalf of the internet retailer.