The U.S. Postal Service is aggressively slashing prices to attract big e-commerce companies in time for the holidays, aiming to steal business from both FedEx Corp. and United Parcel Service Inc.

Over loud protests from its rival delivery giants, the Postal Service won approval from its regulators in August to lower prices by as much as 58% on certain Priority Mail packages for customers shipping at least 50,000 parcels a year.

The Postal Service says its prices were too high to be competitive. But in documents filed with the Postal Regulatory Commission, both UPSand FedEx say the agency is taking advantage of its status as a near monopoly to unfairly snag a bigger piece of the e-commerce pie.

The price cuts "do not reflect a minor cost-related adjustment in the postage that Grandma will have to pay to send a sweater to young Johnny," wrote FedEx in its filing with the commission in July. "What USPS is proposing is an aggressive push to gain market share in the fast-growing business of e-commerce."

A number of e-commerce shippers are considering or have decided to use the Postal Service because of the price change, said Rob Martinez, president of Shipware LLC, a shipping strategy consultant and auditor. "A lot of shippers are going to take another look at the Postal Service."