If you open up a department storecredit card account to take advantageof an introductory offer for 20% offyour initial purchase, you may beopening up a financial can of worms.Unless you pay off that purchasequickly, interest charges can wipe outany amount you save and thensome. With average interest rates ofalmost 20%, department store cardsrack up interest faster than mostbank credit cards, with their averageinterest rate of 14%. If yousave $500 on a $2500 purchase andpay the balance at the rate of $75 amonth, it will take you 3 years to payoff the entire amount. And you'll pay$675 in interest along the way, 35%more than the money you savedwhen you bought the item. That'smoney that you'd likely put to betteruse during your post-income-earningretirement days.