Shares of Stamps.com plummeted more than 48 percent after hours Thursday following the shipping company's announcement that they will no longer be partnering with USPS.

"We will no longer be exclusive to the USPS and that's non-negotiable," said the company's chairman and CEO Kenneth Thomas Mcbride on the earnings call. "The USPS has not agreed to accept these terms or any other terms of our partnership proposal. So at this point we've decided to discontinue our shipping partnership with the USPS so that we can fully embrace partnerships with other carriers who we think will be well-positioned to win in the shipping business in the next five years."

McBride says Stamps.com's goal is to position itself for the best long-term outcome as trends play out in shipping.

Stamps.com issued dismal 2019 guidance. The company sees full-year adjusted earnings per share between $5.15 and $6.15, compared to the estimated $10.79.