Shares of curiously named Sun Country Airlines (SNCY -1.39%) -- which is actually based in Minnesota -- popped in morning trading Thursday. The low-cost passenger and cargo airline surged 12.2% through 11:30 a.m. ET after announcing an expanded relationship with Amazon.

Going forward, Sun Country will operate up to eight more Boeing 737-800 cargo jets in service of the e-commerce giant.

A new dawn for Sun Country stock?

As Sun Country explained in its announcement, it already runs a dozen Boeing jets for Amazon. The companies' new Amended and Restated Air Transport Services Agreement will grow the size of that fleet by two-thirds, and also extend the companies' partnership through at least 2030 -- with an option to extend it out as far as 2037.

It almost goes without saying (though Sun Country said it anyway) that "Amazon is an extremely important customer to Sun Country," helping to smooth out the company's earnings in non-peak travel seasons. Sun Country first landed the contract to run shipping operations for Prime Air jets in December 2019, flying 10 of Amazon's jets. The operation has grown slowly over the past five years, and today's expansion marks the first significant expansion of the company's cargo business since 2019.

Is Sun Country Airline stock a buy?

Sun Country didn't specify the financial impact of its expanded Amazon partnership -- but I think I can guess. Last year, cargo operations contributed just under $100 million to Sun Country's $1.05 billion in annual revenue -- roughly 10% -- according to data from S&P Global Market Intelligence. Expanding the fleet by two-thirds should logically shift Sun Country's business about 7% toward cargo, and grow revenue about 7% as well.

That's not necessarily good news, however, since cargo operations haven't been profitable for the airline for the past two years. Despite the effort to diversify, Sun Country still makes all of its money from its passenger airline service. Perversely, today's good news could actually end up making less money for Sun Country -- and make the stock look even less like a buy.