THE U.S. POSTAL SERVICE has asked the Postal Rate Commission to support a proposal that would correct an error in its nonprofit periodical rate schedule. The error, discovered in January, forced nonprofit mailers to pay higher than commercial rates to mail publications to their members. Large mailers may have paid hundreds of thousands of dollars in excess postage. The USPS wants its Board of Governors to authorize a refund program retroactive to April 9. But Neal Denton, executive director of the Alliance of Nonprofit Mailers, said that nonprofit mailers would like to see the refunds retroactive to January.