Amazon.com Inc. will begin delivering packages on Sundays in the nation's two largest cities later this month with an unlikely partner—the U.S. Postal Service.

The marriage of one of the country's most successful enterprises with one of its most troubled underscores Chief Executive Jeff Bezos's ambitions to weave Amazon more deeply into consumers' lives. The nation's largest Internet retailer has begun same-day grocery delivery in some cities, and is developing smartphones and a set-top box.

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