As if the 29 cents for first-class U.S. stamps wasn't expensive (or awkward) enough, the cost of mailing a letter will climb to 32 cents shortly after the New Year rolls around. The independent Postal Rate Commission today approved the 10.3 percent increase -- part of a set of hikes to infuse the cash-strapped post office with an additional $4.7 billion in 1995. But the commission frowned on a Postal Service request to raise all rates by the same percentage: Instead of charging a quarter for each post card or extra ounce of regular mail, as the post office wanted, post cards will be 20 cents -- up a penny -- and extra ounces will stay at 23 cents. The downside: second- and third-class mail -- including magazines and newspapers -- go up 14 percent, Parcel Post jumps 18 percent and the special library rate increases a painful 70 percent. BTW: The flat first-class stamp rate debuted in 1885 at 2 cents an ounce, cleared 10 cents in 1974 and rose from 25 cents to the current 29 cents in 1988.