Some iconic publications by charities like Consumer Reports and Guideposts may decide to publish less frequently in print because of unexpectedly large postage increases due to take effect next month.

Under the new rates, which will take effect April 26 if approved by the Postal Rate Commission, Guideposts will cost nearly 10 percent more to mail while postage on Consumer Reports and its three spin-off publications will increase by 8.5 percent to more than 16 percent.

Stephen Kearney, executive director of the Alliance of Nonprofit Mailers, which represents about 300 organizations, said he expected some publications would go online only or reduce their frequency because of the postage increase.

The hardest-hit publications were those like Consumer Reports, which are light on advertising, he said. “This really hurt certain really important nonprofits,” he said.

At Consumer Reports, which has 4.7 million subscribers, Meta Brophy, director of procurement operations, said she and her colleagues were suffering from “sticker shock.” The economics of the publications, she added, “changes greatly. This could be a tipping point away from print.”

Controversial Increases

Overall postal increases have been a subject of great contention in recent weeks. On March 6, the Postal Rate Commission told the U.S. Postal Service it had concerns about rate increases it had proposed and sent the plan to increase rates back for more information. But in an announcement Thursday, the Postal Service said it would not change its decision to charge higher rates for publications that have little advertising or are otherwise so thin they don’t weigh much.

While the Postal Rate Commission must still approve the new rates, Mr. Kearney said that there is little the Alliance of Nonprofit Mailers can do at this point to stop the increases beyond asking the Postal Service to explain its approach to rate setting. “This is a done deal,” he said.