The stock was down nearly 3 percent by the close. It regained some ground since the morning, when it fell about 7 percent.
The online shipping services company warned in its quarterly SEC filing that it could see a decline in revenue if its arrangement with USPS was altered.
"While we believe that this agreement is mutually beneficial to the USPS and to us, there is a risk that renegotiation is unsuccessful and leads to materially less favorable terms or that the USPS decides to not renew one or more of these financial compensation arrangements," Stamps.com said. "In such case, our revenue and operating results will be materially affected unless we are successful in timely replacing the lost revenue with similar compensation from other potential partners."
The USPS has service agreements with private companies where they can receive special rates. However, as Stamps.com noted in the filing, the USPS can always decide to renegotiate, discontinue or terminate those agreements.
Prior to this disclosure, Stamps.com saw its stock price increase by 36 percent in 2018.