​Walmart takes aim at Amazon's Prime Day sale

Last Updated Jul 13, 2015 6:43 PM EDT

NEW YORK - The online price fight -- as well as the bickering -- is heating up between Walmart ( WMT) and Amazon (AMZN), and it's not even the holiday shopping season.

About a week after online king Amazon announced a sales bonanza for Prime members set for Wednesday that it touts as bigger than Black Friday, Walmart counterattacked. It has lowered the threshold for free shipping for online purchases to $35 from $50 for at least 30 days, and on Wednesday it will offer discounts on thousands of items online.

Amazon's highly promoted sale, which it calls "Prime Day," is to commemorate its 20th anniversary this week. The sale is aimed at its Prime members, who pay a $99 annual fee for free shipping and other benefits.

"We've heard some retailers are charging $100 to get access to a sale," Fernando Madeira, president and CEO of Walmart.com, wrote in a blog post. "But the idea of asking customers to pay extra in order to save money just doesn't add up for us."

Amazon took the opportunity to fight back.

"We've heard some retailers are charging higher prices for items in their physical stores than they do for the same items online," said Greg Greeley, vice president of Amazon Prime in a statement released a few hours after Walmart's blog post. "The idea of charging your in-store customers more than your online customers doesn't add up for us, but it's a good reminder that you're usually better off shopping online."

Greeley pointed out that nonmembers can still partake in the deals by signing up for a 30-day free trial of Prime.

The moves show how Amazon is hoping to attract more customers to its Prime membership ahead of the key shopping periods of back-to-school season and the crucial holiday shopping period. But Walmart has been learning to respond.

Ravi Jariwala, a Walmart spokesman, noted, "If you've shopped at Walmart.com, you'll know that every day is a special day where everyone has access to the same low prices we offer."

Walmart, based in Bentonville, Arkansas, has stepped up its game against the Seattle-based online sales leader. In May, it announced it will be testing an unlimited free-shipping service for an annual fee of $50. The items will arrive in three days or less.

Walmart also offers a grocery delivery and pickup service in five markets. But the unlimited shipping program marks a substantial commitment and underscores how serious it is about accelerating the growth of its online business, which has seen a sales slowdown.