Letters from our readers routinely express aggravation with ever-changing credit card rules and unexpected fees. The queries and answers below can help you make the most of the cards you hold and avoid the latest issuer tricks.

Q: Can I transfer a balance from an existing card to a new rebate card to earn additional rebate points?

A: Rarely. Most rebate cards award points only for new purchases. Some cards will only let you earn points on transfers during special promotions, but these will have strict limitations.

Q: I love a good deal. To take advantage of low introductory rates, I've opened and closed a dozen accounts in the past three years. Will that hurt my credit rating?

A: There's nothing wrong with chasing teaser rates, but it's important to close your old accounts and make sure your credit report reflects that they were "closed by customer." Otherwise you could be denied a loan because you have too much available credit. In addition, some credit card issuers have begun to screen out people who jump from card to card, so you might be turned down for a card at some point. Just in case, favor cards that give you a low rate even after the introductory period.

Finally, keep an eye on the fine print. A few issuers have tried to impose account-closing fees, or charge a fee for using a convenience check to transfer your balance.

Q: My balance includes purchases, cash advances and balance transfers, each of which carries a different interest rate. The bank applies my payment to the transferred balance first because it has the lowest interest rate. Is there a way to apply the payment to purchases or cash advances instead?

A: Your bank's policy is common -- there's no federal or state law that requires creditors to allocate your payment in any particular manner. The best way around the practice is to transfer your balance to a new card with a low interest rate for balance transfers. Then use a separate, low-rate card for new purchases and cash advances and pay off the card with the higher rate first.

Q: I signed up for a card with a 7.9% rate, and later I got a solicitation for the same card with a 5.9% rate. Can I get the lower rate?

A: Go for it. Because it costs card issuers a lot more to acquire a new customer than to retain an old one, the lower rate is probably yours for the asking.

Q: I mailed in my credit card payment on time, but the bank says it arrived late. Can I do anything to avoid a late fee and interest?

A: Card issuers used to charge a late fee only if you were late by a full billing cycle. But today you might provoke a fee by paying even one day late. A phone call may be all it takes to remove the charge, especially if you have a clean track record.

Or you can dispute the fee as a billing error by writing the issuer within 60 days of your statement date. (You may be able to bolster your case by finding out when your check cleared.) If the bank won't budge, your options aren't great. You can cancel the card and refuse to pay the fee; if it shows up on your credit report, you'll want to add a notation that you dispute the entry. It might be easier to cough up the fee and let it go.

But that can make matters worse, says Robert McKinley, president of Ram Research Group, because some banks are switching customers who pay late more than once to interest rates as high as 21% to 26%. One way to avoid the problem altogether is to use an electronic bill-paying service. Most guarantee that your payment will arrive on time if you send instructions four or five business days before the due date.

Q: I never got my bill this month. Am I stuck if I don't pay on time?

A: Unfortunately, yes. According to Federal Reserve regulations, banks must send you a statement at least 14 days before your payment is due. But you're still responsible for paying on time, even if the bill doesn't arrive. If you're going to move, give card issuers your new address well ahead of time.

Q: My card issuer is about to raise my interest rate. Can I ditch the card and pay off the balance over time at the current rate?

A: Usually, yes. Credit card issuers in several states (including Delaware, where many issuers are based) are required to let you pay off your balance at the old rate as long as you close the account to new charges. Even in states where there is no such law, many card issuers offer the same arrangement. But if you use the card after the new rate takes effect, you will have automatically accepted the new terms.

Q: I hadn't used my credit card for about a year when my bank canceled my account. Can a bank do that?