Correcting to show that Prime costs $99 a year, and not a month.

A simple tweak to the free shipping offer for Prime members could make a big difference to Amazon.com Inc. and create an even bigger threat for its bricks-and-mortar rivals, according to The Harvard Business Review.

Unlimited free shipping for Prime members has been Amazon’s AMZN, +0.18% most effective tool in its mission to edge out competitors, but the increased market share comes at a cost.

The company loses an estimated $1 billion to $2 billion on Prime shipping annually, The Harvard Business Review reported, citing Forrester Research. That is up to 11 times more than the $178 million in operating income that the company reported for fiscal 2014.

The HBR compared the Prime shipping deal to Red Lobster’s 2003 Endless Crab all-you-can-eat crab leg dinners, which were so popular that they ended up hurting the restaurant chain’s bottom line. A disappointing earnings report led investors to wipe out $405 million of parent Darden Restaurants DRI, -2.43% market capitalization in a single day, the worst selloff in the company’s history.

Such unlimited promotions attract over-users, leading to outsize losses, said the HBR.

See:The elephant in Amazon’s mail room

To combat high costs while continuing to attract Prime members, the magazine suggested slightly modifying the shipping offer. Offering just 15 free shipments a year for the current Prime rate of $99 a year and charging just $1 for additional shipments, plus adding a $149 a year unlimited-shipping option would minimize the risks of overuse.

If implemented, such changes could create an even larger threat to physical storefronts.

“It’s time for brick and mortar retailers to be on the lookout,” said the HBR. “With an enhanced Prime program, focus on same day delivery, and now its own holiday, Amazon is clearly setting its sights on fully invading land-based retail territories.”

Read: What not to buy on Amazon