The only union to reject the Postal Service's proposed $4.8 billion increase in pay and benefits has accused unions that signed the tentative agreement of accepting a "piece of garbage."

"Even a casual examination of the . . . 'contract' will prove to you that it is the worst agreement ever negotiated by postal leaders," said James J. LaPenta, Jr., chief spokesman for the Mail Handlers Division of the Laborers' International Uniion. His message was delivered in a July 22 letter to local union leaders.

The Mail Handlers, representing 63,000 workers, was the only bargaining unit to walk away from the table Tuesday without an agreement with the Postal Service. Negotiators for the American Postal Workers Union and the National Association of Letter Cariers, which represent about 500,000 postal workers, tenatively okayed the new three-year contract. A fourth union, the National Rural Letter Carriers Association, signed a separate agreement, the details of which have yet to be made public.

LaPenta's group is submitting its unresolved differences to binding arbitration under the provisions of the 1970 Postal Reorganization Act.

APWU and NALC leaders said the pact they signed would yield about a 10 percent pay raise over the next three years, and would continue an "uncapped" cost of living allowance that could kick in as much as $3,619 if inflation persists at 1978-1981 levels. The unions' leaders called the proposal "a good contract."

However, LaPenta, whose organization has been at odds with the two larger groups, said the temporary agreement will bring less than a 3 percent per year increase in wages -- compared to first year increases of 11 percent won by other major collective bargaining units this year.

The temporary agreement also contins weaknesses in the areas of health and safety and productivity pay adjustments, said LaPenta, whose letter is expected to be widely read by APWU and NALC members, as well as members of the Mail Handlers.

The missive is expected to increase the difficulty of selling the temporary pact to ACLU and NALC members, who will be voting on the agreement next month.