In a substantial enlargement of the U.S. Postal Service's franchise to deliver communications during an era of technoligical revolution. President Carter endorsed plans yesterday from electronic mail - a decision certain to attract bitter opposition from some private businesses.

Postmaster General William Bolger earlier had sought administration backing before launching a $1.7 billion electronic delivery system that could take two decades to develop fully. Yesterday he hailed the White House decision as providing the opportunity to offer improved mail services to the public at lower costs.

An initial Postal Service offering would be called Electronic Computer Originated Mail (ECOM), allowing companies with computers and large amounts of regular billings to be tied to a Postal Service computer; bills and invoices would be printed out at local post offices, automatically stuffed in envelopes and then put in normal channels for delivery to customers.

In a statement of policy issued by the White House late yesterday after Carter aides conferred with key congressional leaders, the president said the nation's post office should be allowed to use new telecommunications systems to feed messages into the noral mailstream for delivery by Postal carriers.

At the same time, Carter said the Postal Service should be prohibited from offering "end-to-end" electronic services - such as a direct technological link with a customer's offices.

In essence, the joint White House-Postal Service agreement will allow post offices to take part in the new communications age but without a strong competitive edge over private firms.

Postmaster General Bolger accepted eight conditions mandated by the White House to permit delivery service competition.

Among the conditions were continued opposition to any legislative or regulatory efforts aimed at restricting competition; a requirement that electronic mail operations not be subsidized by tax money or revenues from other postal services; establishment of the electronic mail system as a separate entity for accounting and ratesetting purposes; a ban on building a separate transmission network, thus forcing the U.S. Postal Service to purchase electronic services from common carriers; and Federal Communications Commission regulation of the transmission portion of overall rates, leaving postal delivery rates under the Postal Rate Commission, with a review after five years.

Communications companies, including American Telephone & Telegraph Co. and such relative newcomers to the business as Exxon and Xerox, have opposed an electronic delivery role for the post offices, charging that even the limited franchise proposed by Carter yesterday will curb ultimate competition because the Postal Service is so huge (it has 40,000 post offices to use as delivery centers).

Communications companies are expected to touch off a legislative battle over the issue. Franklin Julian, tele-communications policy director for American Telephone and Telegraph Co. said last night. "We are naturally disappointed that the Post Office is being encouraged to provide a kind of service which we feel private industry is able to do."

Vincent Sombrotto, president of the National Association of Letter Carriers, said his union would do everything it can "to persuade the Congress to eact legislatio which will put the president's recommendations into law."

But Carter said that so long as physical delivery through the mails is the primary method of communications to a large segment of the population, the Postal Service should tak advangage of electronics to imporve its services. Moreover, it will take years to develop.

The White House cited Postal Service estimates that the average price to customers of each electronic message eventually would be 9 cents or 10 cents in the 1985-1995 period (using 1979 dollars).