What happened

I've got good news and terrible news for Stamps.com (NASDAQ:STMP) shareholders today.

The good news: Q4 earnings were just fine. Customer churn rates declined to a minuscule 2.9%, while average revenues per customer and total revenue for the quarter both surged 29% year over year, and profits climbed a modest 6%.

The bad news: None of that matters, because Stamps.com is terminating its exclusive deal to sell postage for the partner that accounts for the vast majority of its business, the U.S. Postal Service. Stamps.com stock is down 55.7% as of 11:35 a.m. EST in response.

A stamp collection.

Stamps.com: Proof that not all stamps appreciate in value. Image source: Getty Images.

So what

In a postearnings conference call explaining why Stamps.com management felt compelled to predict a 5% decline in sales this year (versus last year) and a much more significant 63% decline in profits, Stamps.com CEO Ken McBride explained that "our revenue share partnership [with the U.S. Postal Service] has now ended."

Arguably worse, this appears to have been a decision initiated by Stamps.com -- not by the USPS. In a lengthy speech, McBride laid out his thinking about where the parcel delivery market will be heading over the next five years. Simply put, Stamps.com feels USPS doesn't give Stamps.com users the best prices or the best service, so the company felt compelled to cut exclusive ties with the post office in order to provide its other customers with other, better shipping options in addition to USPS. The CEO reassured investors that Stamps.com users can still buy postage for shipping via USPS if they want to. As for Stamps.com itself, though, it will be losing the high-margin "commission component" of such USPS revenues.

What McBride did not clearly explain was why he chose to drop this bombshell at this particular time --abandoning a relationship with Stamps.com's biggest partner before securing enough similar relationships with other shippers to replace the revenue and profit that will be lost.

Now what

The simple, hard truth is this: The exclusive relationship with the USPS, which permitted Stamps.com to grow by leaps and bounds in years past, can no longer be depended upon to power that growth in the future. Stamps.com may have widened its potential market, but it's blurred its future prospects.

Investors can hardly be blamed for thinking that's a bad thing for the stock.