Although Christmas is over, there are only four shopping days left for connoisseurs of expensive cars and luxury furs, as well as for devotees of booze and beer.

On Tuesday, Jan. 1, a set of new excise taxes, enacted as part of the budget agreement last fall, go into effect with a sting that will be felt by Joe Sixpack as well as the jet set.And many merchants, using newspapers, radio and television, are letting the public know that prices are going up, up and up.

Signs on liquor store windows scream out "Prices Go Up Jan. 1, Buy Now" and a Cadillac ad blares "Beat the Luxury Tax - you have until Dec. 31, 1990, to save thousands."

The fancier liquor store owners report the well-to-do stocking up on some of the high-priced items, such as expensive wines and liquers, but few report a run on everyday beer.

Dealers of high-priced cars report a jump in sales in the past couple of months but aren't that sure it can be attributed to the new tax.

Stephen Smythe, executive vice president of Euro Motors in Bethesda, Md., which sells Mercedes-Benz that range in price from $28,000 to $90,000, said that initially there was no response to the prospect of an increased tax.

But Smythe said, "Since November, we've enjoyed a little bit of an increase," adding that sales are running about 20 percent above the comparable period in 1989.

Two taxes, the gasoline tax and the aviation tax, went into effect Dec. 1.

The fuels tax is 5 cents a gallon, raising the federal gasoline tax to 14 cents a gallon and the diesel fuel to 20 cents a gallon. The aviation tax increases the tax on a ticket from 8 percent to 10 percent.

The taxes that go into effect Jan. 1 are on alcoholic beverages, tobacco, gas guzzlers, telephones and luxury items.

The luxury tax imposes a 10 percent surtax on the portion of the retail price above $30,000 for cars, $100,000 for boats and yachts, $250,000 for private planes and $10,000 for jewelry and furs.

As an example, on a car that retails for $50,000, the surtax is 10 percent of $20,000 or $2,000.