Credit cards used to be straightforward, like a hand of five-card stud. But then came rebates, balance transfers and an assault of penalties for day-late and dollar-over-the-limit transgressions. Today, keeping a low interest rate, avoiding penalties and making sense of rebate programs takes a rule book worthy of Hoyle.

Letters from our readers routinely express aggravation with ever-changing rules and unexpected fees. Here's a sample of one of the questions about dealing with lender goof-ups.

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

Answer - 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.

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 Re-search Group, because some banks are switching customers who pay late more than once to interest rates as high as 21 percent to 26 percent.

One way to avoid the problem altogether is to use a bill-paying service, such as Quicken or Check-Free. Most guarantee that your payment will arrive on time if you send instructions four or five business days before the due date.