The stock price of the El Segundo, Calif.-based company plummeted nearly 50% nearly three hours after markets closed Thursday, falling to as low as $102.05.
Stamps.com had already taken a beating during trading on Thursday after it unveiled, with little explanation, a gloomy earnings forecast for 2019.
In a 5 p.m. conference call with analysts, Kenneth Thomas McBride, Stamps.com's chairman and CEO, revealed the reason for the dismal projections, detailing the split with the USPS.
The breakup came after Stamps.com said it no longer wants to be exclusive with the Postal Service, terms that the postal service refused to accept, McBride said told analysts.
"We will no longer be exclusive to the USPS and that's non-negotiable," McBride said on the earnings call.
Stamps.com may pay a steep price for seeking independence from the USPS, with the online shipper now projecting earnings per share of $5.15 to $6.15 and revenue of $540 million to $570 million.
That's well below analyst estimates gathered by FactSet, which had pegged Stamps.com's earnings at $10.79 per share based on just over $689 million in revenue.