I paid off the balance on my GM Flex Earnings Card from HSBC bank a couple weeks ago.  Today I double-checked the account on-line, and was surprised to see my balance was not $0.  It was $6.66.  I scratched my head for a bit, and then realized what happened.  When I paid the balance a couple weeks ago, it was the balance from my last statement, not the "pay-off" amount.  The pay-off amount is higher than the last statement balance, because interest charges are applied daily.  If I paid the statement balance the day it was posted, I would have been paid in full.  In actuality, I paid a few days after it posted, thus I owed additional interest for those days.

 The lesson is that in general, you will have to pay off a card twice.  Once for the statement balance, and again for any residual daily interest that will be calculated on your next statement.  With my cynical eye, I see another ploy to trip up consumers here.  Issuers could easily provide a pay-off amount any time you look at your account on-line.  My bank provides this for each loan I have with them.  I believe issuers choose not to do this, because this is an opportunity to bring in more late fees.  Here's how that would happen.  Most consumers would pay the balance due just like I did, and assume they have a zero balance.  Some would miss the fact that they need an additional payment, and would miss this payment and owe a fee.  This could spiral into quite a problem if multiple months pass without you detecting the problem.  Fees would grow, and your issuer would report your delinquency to the credit reporting agencies, affecting your credit score.

 So, when you finally pay off a balance on a credit card, be sure to check the account later to see if you have additional interest that you must pay.  Only when you see a zero balance on the next statement can you be sure that the account is truely paid off.