Wal-Mart says it will test a free shipping program, the latest sign that the world’s largest retailer is stepping up its lagging e-commerce efforts to better compete for online shoppers.

The program, which will be available on an invitation-only basis beginning this summer, pits Wal-Mart against Amazon’s massive Prime program. For $50 per year, a fee that easily undercuts the $99 Prime membership, Wal-Mart subscribers will have free shipping on all their orders, which the retailer promises will arrive in three days or less. (Prime customers typically receive two-day free-shipping.)

“We’ve heard from customers that they want shipping that is predictable and affordable,” said Ravi Jariwala, a Wal-Mart spokesman.

It’s easy to see why Wal-Mart is experimenting. Expanding its online business is critical to the retailer’s overall growth, and the big box chain has said it will plow at least $1.2 billion into e-commerce initiatives this year. The company pulled in $12.2 billion from online sales last year. While that represents a 22 percent increase, it is only a tiny fraction of Wal-Mart’s $485.7 billion in revenue. And it is less than what Amazon made on selling items during its most recent quarter.

With more than 4,500 Wal-Mart supercenters dotting every corner of the U.S., the retailer also knows its fleet of mega-stores can’t grow much more, making its online strategy even more important. So it is testing a variety of new online offerings, including allowing customers to order groceries online and pick it up at a store.

But the subscription shipping model may not be a good fit for Wal-Mart.

Its chief rival will be Amazon Prime, which offers its customers convenience — not necessarily low prices. They are signing up for the comfort of knowing that they can quickly order diapers, batteries, books or millions of other items and they will arrive quickly.

But the Wal-Mart shopper is typically focused on rock-bottom prices above all else. Would that type of shopper pay $50 for the convenience of receiving their online purchases quickly? Couldn’t they save the cost of the membership fee by placing larger orders, since Wal-Mart already offers free shipping on orders over $50?

It doesn’t help that many shoppers are already locked into Amazon Prime’s ecosystem. While Amazon does not disclose how many members it has in its Prime program, researchers have estimated it is in the tens of millions. It will likely be difficult for Wal-Mart to lure away those shoppers, especially since Prime members can also listen to an unlimited amount of streaming music, watch movies and TV shows, and store their photos online — all services Wal-Mart currently doesn’t offer.

“I think that it may appeal to some segment of Wal-Mart’s existing shoppers, but I don’t see it taking big business away from Amazon,” said Paula Rosenblum, a retail analyst at RSR Research.

Meanwhile, an industry newcomer, Jet, could also present tough online competition for Wal-Mart. Jet charges $49 for membership, but it says that its prices will be lower than Wal-Mart’s. (Jet is still in beta mode and won’t debut to a mass audience until later this year, so it remains to be seen whether it can regularly make good on that pledge.) Jet has already drummed up enormous interest, including 350,000 shoppers who signed up for early access to shop the site for free.

Wal-Mart’s limited mix of merchandise could also be problematic. The company said that people who sign up for the service will initially be able to choose from only 1 million of its top-selling items, rather than the roughly 7 million items on its Web site. That doesn’t stack up well against the 20 million items that are available for Amazon Prime, or even the 10 million items that are to be available on Jet.

Wal-Mart is testing other ways to improve its online sales, such as a grocery pick-up service that allows customers to place an order online and pick it up in a drive-thru set-up. So even if the unlimited free shipping membership doesn’t take off, it has other arrows in its quiver as it tries to invigorate its e-commerce business.