The payment page and the followup invoice I received after paying don't explicitly state the currency, only "$10".
Plenty of different countries use the "$" symbol, and many websites adapt the billing based on where the user is, so it wasn't clear to me whether I was being charged 10 USD or 10 NZD (I'm in NZ)
Great service by the way, happy to pay the 10 USD; it just needs to be more clearly stated on the pricing and payment pages.