Most Merchants require credit or debit cards to have address verification. Your bank may need to activate your card for internet/phone purchases. You may have tried to order from outside of your home country. Some banks require you to activate the card for international purchases even if ordering on the internet. If you placed a lot of orders you may be over your daily authorization limit. Some merchants charge you an authorization and a purchase which looks like two charges because the merchant does not know how to merge the transaction. The authorization is usually removed in about a week.

To correct some people above

In person US debit card with a Visa or MasterCard symbol can be used two ways at some merchants

Use the Credit button and you usually sign for the purchase.
Use the debit button and you enter your pin at the pos terminal

Online and by phone it can only be used as a Credit Card and you should never give anyone your pin number. All ways come out of your checking account with no delays using a debit card. My experience. You are also protected with debit card purchases. The key is notifying the bank fast.

I see that it is!

I can help

If you are getting an error saying that this security number is invalid, you either haven't entered the CVV2 number correctly or you have a bogus card.

When you make a purchase, you swipe or give your card to a cashier. This initiates an electronic call to the credit company [Visa] who will then approve or decline the transaction based on your available credit. If approved, you will usually be asked to sign a receipt stating that you will pay the transaction. This receipt is counted into the companies daily revenue and then deposited in the bank where it or an electronic copy of it is sent to the credit company [Visa], similar to how you get a check that you write back after it is cashed. This signature is kept on file in case you don't pay, as it is a legally binding contract.

Once Visa has the signature, they send the payment to the company that you made the charge at [the delay of some companies sending the receipt back to Visa is what causes some things to pend for a few days before posting to your account]. You are then billed on your next statement. If you pay the bill by the due date, you will not pay any interest. If you let a charge stay on your bill for over a month, you begin getting finance charges on it until you pay it. This means that you end up paying more for the object then it was originally worth.

If you continuously pay your bill on time, you will recieve raises to your credit line once or twice a year. If you do not pay your bill on time, you will be fined and have interest charges added to your account.

ADDED: I saw your update that said you get $1000 a month, and wanted to let you know that it does not work as a limit per month. $1000 is your limit overall, meaning if you go over 1000, you will not have ANY credit available until you pay your balance down, and you will continue to get Overbalance charges each month until you do so.

Be careful with these store cards… they are expensive to maintain.

http://creditcardforum.com/store-issued-cards/728-macy-s-credit-card-warning.html