One of the most durable of Christmas traditions is the daily pile of mail: welcome cards and parcels, certainly, but more often catalogs, subscription offers and pleas for contributions--in a word, junk mail.

But the message in the mail this year is very non-traditional. Eleven years after the Post Office was reincarnated as the U.S. Postal Service, the keys to its future, and to the price of postage stamps, are a nine-digit zip code and more junk mail.

"We don't have any such thing as 'junk mail,' " objected Postmaster General William F. Bolger in an interview. "We call it advertising mail."

Last year the Postal Service handled 113 billion mailed items, perhaps half the mail on the planet, delivering an average of 1,412 articles to each of the nation's 80 million households. Only 6 percent of that was personal cards, packages and letters. The rest was business mail, such as bills and orders, and advertising mail.

Now the Postal Service is the nation's third biggest advertising medium, after television and newspapers. Householders bought $110 billion worth of goods through the mail last year, according to the Direct Marketing Association (DMA), and the trend is upward.

Partly for that reason, the Reagan administration is discussing further deregulation of the mails, opening the possibility that private companies might compete with the Postal Service even in delivering personally addressed letters. It is "still in the talking stage," according to White House officials, with no immediate prospects for legislation.

Delivery services already compete strongly on parcels and other kinds of mail. But private firms are cautious about total deregulation, for replacing the Postal Service in remote rural or dangerous inner-city routes would be a major undertaking.

"If you think the mails are fouled up now, you should oppose deregulation with your dying breath," said a congressional staff member close to the issue.

In fact, independent surveys show that 95 percent of all zip-coded first-class mail that should be delivered overnight does in fact get there the next day. For second-day mail, the figure is 88 percent and for third-day mail 90 percent.

"Even if it you make only a 1 percent error, that's 3 million pieces of mail going astray every day," Bolger said. "People remember that and forget the rest."

Bound by the Postal Reorganization Act of 1971, as well as tradition, to deliver all that mail at uniform prices to every square inch of American territory, the service must pay for clerks, sorters, carriers, buildings, uniforms, vehicles, gasoline and so on whether it delivers one letter to Aunt Minnie or 150. So the flood of catalogs makes the whole task do-able, according to Bolger.

If more mail moves faster, costs can be shared among more pieces, which means stamp prices do not rise. Predictable rates, essential for advertisers' planning, is Bolger's goal.

"The total system is helped by volume increases anywhere along the line," Bolger said. "We've increased the volume by 30 billion pieces since 1970, we're delivering mail to 18 million addresses that didn't even exist then, and we're doing it with 70,000 fewer employes."

He is also doing it at a profit, although not for long. The Postal Service ran an operating surplus of about $700 million last fiscal year, plus a bookkeeping profit from unused reserve funds that brought the total to $1.08 billion. But the plan is that this excess will cover losses expected as future costs rise, and that another rate increase will be needed in 1984. Bolger hopes to make the cycle routine for everybody, including Congress.

Large mailers love the idea and oppose deregulation. "The first thing to go if you got rid of the monopoly would be the single-price, first-class stamp," said the DMA's Richard Barton. Rates would have to vary depending on distance and accessibility, he said, and mass mailers would not be able to function. To achieve his stable rate cycle, Bolger wants to move even more mail and move it faster, which he says means more automation that in turn means the nine-digit zip code. Last year the idea was delayed until next October by a horrified Congress.

All sides agree that the Postal Service bungled its first attempt to sell what it calls "zip plus four." Congress worried that the public, already assaulted by Social Security, license and telephone numbers, never would be able to remember nine digits. The outcry drowned out Bolger's insistence that the system would be voluntary and used mainly by mass mailers.

A recent General Accounting Office report criticized the Postal Service's use of "uncertain, sensitive and soft" numbers to justify the $2.76 billion investment it wants to make in machines to read printed zip codes, machines the report said have not been very well tested.

There are "some risks involved that cannot be assessed at this time," the report said, including the degree to which major mailers will overhaul their addressing equipment.

Instead of earning a 20 to 25 percent return each year as the Postal Service estimated, the report said, a return of 16 to 18.6 percent is more likely. "What the devil is wrong with that?" Bolger said. "If that's the worst case, I'll take it."

The Postal Service's Board of Governors recently approved a price cut to get major business mailers to use the superzip. Unless Congress balks again, new machines will be installed gradually to begin the new system next October for business letters.

"Opposition is illogical," said Richard Coughenour, chairman of the New Postal Policy Council, a group of major business mailers. "Householders won't have to use it. It'll be on the mail that comes to them."