That detail shows the huge size of one of its lesser known services. Helping third-party businesses sell in its online store generates extra fees and adds to the number of products available to shoppers beyond just what Amazon sells directly.

Over the past two years, the fulfillment program has seen a big growth spurt as more merchants join. The number of sellers using the service grew more than 50 percent in 2015 after a 65 percent rise a year earlier, according to Amazon, which did not disclose the number of merchants involved.

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