If the term is free in most cases the apartment owner requires an advance / deposit of 10% to 50% of the total price of the accommodation to confirm the reservation - to make sure you come or as a fee in case of no-show. The deposit is not refundable unless otherwise agreed.
You can send the deposit to the owner's bank account (European SEPA payment - fee depends on the bank - usually 1 EUR) through your bank or through the service PaySend or by Post service to the owner's address in Croatia or by fast money transfer via the more expensive Western Union service (fee approx. € 15 for amounts up to € 100, fee approx. € 20 for amounts up to € 200)