Consumers were warned Wednesday by the Postal Inspection Service not to fall prey to a nationwide mail order scam that is encouraging the underpayment of first-class postage.

Promoters of the mail-order scam are selling an information package at a cost ranging from $5 to $100 that describes how to mail first-class letters for 2 cents instead of 25 cents.Roy W. Geffen, postal inspector in charge, said the scheme is designed to bilk the postal customer out of money for inaccurate and out-of-date information. He said postal inspectors have filed administrative and criminal complaints against the promoters.