The ZIP code, that string of numbers bringing up the rear of every American address, is a quarter-century old Friday. "We'd certainly describe it as a roaring success," says John G. Mulligan, senior assistant postmaster general for mail processing and delivery. The Postal Service handles more than double the mail today that it did when the codes were introduced, but with only 15 percent more people, Mulligan said. And the ZIP system is a big factor in that saving. "No direct calculation of the exact savings has been made; it's hard to separate the effect of the ZIP code from other things. But believe me, I think it's up there in the several billions of dollars," Mulligan said in an interview. In 1963 the Post Office carried 67.9 billion pieces of mail, sorted by human beings at a maximum rate of 800 items an hour. The agency handled 153.9 billion pieces last year, using machines sorting at a rate of 27,000 to 35,000 pieces an hour.