How do I change the currency prices are shown in?
Use the currency selector in the header to switch between US dollars, Canadian dollars and Mexican pesos. It changes what you see, not what you are charged.
Converted prices use live exchange rates and are there for comparison. The charge itself is made in the currency the captain set, and checkout tells you which that is before you pay.
The selector remembers your choice, so the site stays in your currency as you browse. The rates come from a live feed and update through the day, which is why a converted price can shift slightly between visits — the captain's price has not changed, the exchange rate has. At checkout the estimate disappears and you see the exact amount in the currency the captain set; that figure is the one that matters.