It’s a scam. If you reply they will ask you for a credit card number or bank account number to pay for shipping and handling and insurance. They will spend all of the money in the account. They will never send you a gift card. If you visit the website in the message it will download spyware and viruses and heaven knows what else.
Delete the message. Do not reply to it. Do not visit the website in the message.
This thread
http://800notes.com/forum/ta-46d2613af143609- ... t-buy-gift-card
has almost 2000 posts, people like you who got the e-mail and people like me telling them it’s a scam.