Walmart is going after Amazon Prime customers with a new free shipping offer

Walmart is aggressively going after Amazon Prime customers with a new free shipping promotion.

The retailer is offering a free month-long trial of its unlimited two-day shipping program, called ShippingPass, to new customers.

Walmart is also giving current ShippingPass members an extra month of two-day shipping, free of cost.

ShippingPass was designed to rival Amazon's Prime membership, which also offers unlimited two-day shipping - but for a $99 annual fee.

ShippingPass costs $49 a year, by comparison.

Walmart just recently shortened the program's shipping window from three days to two days, making it a more formidable rival to Amazon Prime.

Like Amazon Prime, ShippingPass gives subscribers access to more than 1 million products online that they can get delivered to their doors within a couple of days.

There's no minimum-order requirements to qualify for the free two-day shipping. The membership comes with free returns online and in stores.

The shipping changes are Walmart's latest bid to take on rival Amazon.

The retailer has been investing billions into its ecommerce business in recent years, but Walmart's online sales are still a tiny fraction of Amazon's.

Walmart's online sales were $13.7 billion in fiscal 2015. By comparison, Amazon's online and electronic device sales in 2015 were $99.1 billion.

But Walmart has been slower than Amazon to offer fast and free shipping for online orders.

Without ShippingPass, Walmart customers have the option to choose between "rush" one-day shipping, which can cost at least $14 per item, or "expedited," "standard," and "value" options, which take between two and seven days and cost about $5 to $8. Walmart offers free pickup at stores.

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.