Amazon.com Inc. (AMZN) agreed to buy online apparel-and-footwear seller Zappos.com Inc. for roughly $850 million in stock and cash, a deal that adds more products and inventory to the world's biggest e-tailer.

Amazon will pay $807 million in Amazon stock to purchase all of Las Vegas-based Zappos. It will also give Zappos employees an additional $40 million in cash and shares.

The deal would be the most expensive purchase in Amazon's recent history, according to FactSet Research. Amazon's priciest acquisition this decade was Audible Inc. - which provided audio editions of books and newspapers - for $280.7 million cash in early 2008, according to FactSet.

Purchasing Zappos marks yet another stage in Amazon's growth through both acquisitions and organic development. Founded as an online bookstore, the Seattle-based company has expanded into a host of retail categories, including electronics, toys and music. The company has also developed the Kindle series of book readers, that buyers can use to download electronic versions of books from Amazon.

Analysts praised the deal, saying it would be mutually beneficial.

"These are two recognized dot-com companies getting together, which only enhances each other's growth potential," Fred Moran, an analyst at Benchmark Capital said. He called it "small but wise."

Zappos doesn't disclose financial details, but the company reportedly recorded $1 billion in gross sales last year.

Chief Executive Tony Hsieh vowed in an email to employees that "we plan to continue to run Zappos the way we have always run Zappos."

Hsieh also said Amazon reached out to Zappos several months ago. A key factor was Amazon's intent to let Zappos continue to build the Zappos brand "as opposed to folding us into Amazon."

The acquisition is one of a series of recent Amazon investments, including the acquisition of visual-product-search company SnapTell Inc. for an undisclosed amount last month. Speculation has also mounted that it's interested in buying online DVD rental service Netflix Inc. (NFLX). A Netflix spokesman declined to comment.

The Zappos management team will remain intact, and the company will operate its brand as a wholly owned subsidiary. The deal is expected to close this fall.

Amazon shares were up 1.3% to $89.90 in after-hours trades.

-By Ben Charny, Dow Jones Newswires; 415-765-8230; ben.charny@dowjones.com

-By Lauren Pollock, Dow Jones Newswires; 212-416-2356; lauren.pollock@dowjones.com

(Scott Morrison and George Stahl contributed to this report.)