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Amazon.com disclosed Friday that it ships more than twice as many items to members who use its Prime subscription service in the United States than to those shoppers who opt for free shipping.

The company is typically opaque when it comes to details about Prime, the $79 a year service that offers free two-day shipping on more than 15 million items. It won’t, for example, disclose how many members the service has. The detail about the Prime shopping data came buried in a press release about holiday sales trends.

To be sure, it’s not particularly precise data point, since the company would only say the units, not dollar amounts, shipped using Prime are twice the number as those sent via its free shipping offer. What’s more, Amazon racks up plenty of sales of items for which consumers opt to pay for shipping.

Still, the detail about how much more Prime members shop offers insight into Amazon’s strategy to add more features to boost enrollment in the program.

Amazon Studios, for example, has been busily building a slate of original programing for its Netflix-like Prime Instant Video streaming video service available for members. And the company also offers a free lending library of more than 350,000 digital books for Prime members who also own Kindle e-readers.

Just last week, Amazon boosted the minimum order price shoppers had to reach before getting free shipping by $10 to $35 in a move that was widely seen as an effort, in part, to drive customers to Prime.

An Amazon spokeswoman declined to disclose any more details about Prime shopping data.

Jay Greene: 206-464-2231 or jgreene@seattletimes.com. Twitter: iamjaygreene