If you book through Autoeurope from the USA site the rates are guaranteed in US$. They have many international sites though and although you can book through any one of them the rates and insurance coverage varies significantly, so you have to be careful. For example the Australian site guarantees in US$ but the insurance deductible option is terrible. The British site guarantees in GBP and has good rates and great deductibles. If you go the car option Autoeurope or Nove is the way to go.