Walmart is extending free two-day shipping next month to millions of third-party products sold over its website, part of the company's aggressive push to capture online shoppers.

The world's biggest retailer already offers the free service for orders of $35 or more on items it sells directly, which has helped it boost sales and keep pace with Amazon.com. Walmart will also let shoppers drop off returns of third-party items at its 4,700 stores, rather than having to ship them back.

The offer represents Walmart's latest move to make its website more appealing ahead of the holidays. It follows a redesign earlier this year, an online partnership with Advance Auto Parts and acquisitions of plus-size apparel site Eloquii and lingerie seller Bare Necessities.

Walmart wants more online customers, who spend nearly twice as much as its relatively frugal in-store shoppers. More than eight out of 10 holiday shoppers will spend the same or more this year, according to a survey from PricewaterhouseCoopers.

More than 90 percent of the 43 million products for sale on Walmart's site are third-party items, according to data tracker Marketplace Pulse. Those merchants will bear the additional costs for the faster delivery. At an investor meeting last week, Walmart's U.S. e-commerce chief Marc Lore said the company needed to boost sales of items outside its core of everyday household goods. Those items are more profitable, Lore said.

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