Stamps.com has pulled out of an exclusive deal with the U.S. Postal Service—sending its stock plummeting more than 50%, or over $100—in a move meant to keep the company flexible as Amazon enters the shipping industry.

CEO Ken McBride said the decision will cause “some short-term pain for us over the next few years,” with revenue possibly slipping as much as 8% this year. According to CNN, analysts’ forecast had projected sales to increase by more than 16%.

Earlier this month, Amazon listed “transportation and logistics services” as a competitive industry on a regulatory filing, signaling that the company…