For 20 years, visitors to upper Grant Avenue in North Beach have been dazzled by the visual overload of Quantity Postcards, one of the campiest stores in the city. The walls are completely lined with postcards, in thousands of designs, ranging from cheesy antiques to the latest styles, though most are of a humorous bent. Prices are 25 to 85 cents per card, and while many are politically incorrect or downright ridiculous, none could be called offensive. Prices are much lower for wholesale purchases (20 dozen or more). The store has a spinoff, Tilt, at 507 Columbus Ave. (between Stockton and Union, 788-5566, www.tiltpix.com), which, besides postcards, also stocks movie publicity photos and posters, buttons, and hundreds of refrigerator magnets. "When I feel down, I walk around and read the magnets," says Tilt owner Bruno Guarini. "There's always one that will make my day." In an area overrun by fine restaurants, these two stores preserve some of the insanity of the old neighborhood.