The sum to be paid on the spot and the currency of payment are indicated in your voucher. Moreover this information is available while booking a transfer. The payment should be made according to the official rate valid on the day on which the transfer service is provided and in that country where the transfer is provided. We do not know what official rate will be valid in the future within the day of transfer service, and it is not possible to accurately forecast it.
But it is possible to book a transfer with a 100% prepayment on the site, and you will not have to worry about currency exchange, rates, etc. on place.