you can bet it's a scam. Legitimate buyers don't ask you to compensate someone (such as a driver). Places where you win a prize don't ask you to send tax money to them. People who buy don't round up and ask you to send them a refund.

Always consider whether the payment you received can be reversed or whether it really has cleared. Another common scam involves money orders or checks from Canada since people think they clear as quickly as American checks.