Donahoe has said the service is losing $25 million every day. His plan would end Saturday letter delivery starting in August but preserve six-day package delivery. Fully implemented, the limited schedule would save $2 billion annually, according to estimates.
The plan caused a stir on Capitol Hill and raised the stakes for a postal reform deal some leaders say will conclude within months.
In their letter Wednesday, lawmakers noted that some medications are sent in small envelopes, or through First-Class Mail, which could hamper their on-time delivery under Donahoe's plan.
The letter was distributed by the Pharmaceutical Care Management Association, which represents pharmacy benefit mangers.