The company says that Amazon is interfering with the shipping industry, and it must make freight deals with postal service providers entering the future.
"One of our unapproved items is that … we will no longer be exclusive to USPS," CEO Kenneth McBride said in an interview with analysts on Thursday night. "USPS has not agreed to accept these terms or any other terms of our partnership proposal."
McBride said Stamps.com chose to terminate its freight partnership with the postal service to "fully embrace partnerships with other carriers that we believe will be well positioned to win in the shipping business over the next five years."
USPS refused to comment on the news.
The newsletter sent shares that doubled to 58% in morning trading before climbing off the low of the day.