If you are laying down serious money, it's best to call them up on the phone and ask questions like voltage etc before buying. Not everything comes with dual voltage power adapters. If you're lucky you'll get a knowledgable sales person who can help you. What are you thinking of getting, and from which online merchant?
Electrical goods usually run on 110V with the US plug. You won't be able to get 240V unless it's something that can be brought for travelling, like shaver, mobile phone charger, etc, since no one's expecting you to bring your toaster on a trip right? Even the variable voltage ones come with the US plug, so you'd need an adaptor for that.