New York (CNN Business) Stamps.com says it is ending an exclusive relationship with the US Postal Service, sending shares of its stock plunging more than 50%.

The company says Amazon is disrupting the shipping business, and it needs to do shipping deals with postal service competitors going into the future.

FedEx FDX UPS UPS Amazon has popularized two-day shipping with its Prime service, and Stamps.com says its customers demand two-day shipping guarantees.and DHL offer those guarantees. The Post Office doesn't.

"One of our nonnegotiable items is that ... we will no longer be exclusive to the USPS," said CEO Kenneth McBride in a call with analysts on Thursday evening. "USPS has not agreed to accept these terms or any other terms of our partnership proposal."

McBride said Stamps.com opted to discontinue its shipping partnership with the Postal Service so it 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."

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