At the selling point number 6 - Claims, you can return the ticket in accordance with Article 4.1 of the Online Ticket Sale General Terms and Conditions, which refer to Article 23 of the General Terms and Conditions of Bus Station Operations.
 
Article 23 of the General Terms and Conditions of Bus Station Operations:
 "Zagreb Bus Station shall, at the request of the customer who cancels his/her trip, refund the amount paid for the ticket, reduced by 10% of the ticket price for handling costs, with no possibility of refund for the bus station service, seat booking and boarding pass:
if the passenger contacts the claims office at Zagreb Bus Station no later than 1 hour prior to departure for domestic public regular transport,
if the passenger contacts the claims office at Zagreb Bus Station no later than 2 hours prior to departure in international regular transport. Outside of the time limits referred to in the preceding paragraphs, the claims office clerk will instruct the passenger to resolve the request directly with the carrier.
 
The provision of paragraph 1 of this Article also applies if:
the carrier cancels departure for a given route, in which case the amount refunded to the passenger is not reduced by 10%, as well as the amount paid for the bus station service, seat booking and boarding pass:
if the carrier for a given route has reported a delay of 30 or more minutes from the scheduled departure time. The provision of this article applies only to tickets sold at Zagreb Bus Station selling locations/ticket offices.
 
The passenger may replace the ticket for another departure of the same carrier by contacting the claims office at Zagreb Bus Station no later than 1 hour prior to departure in domestic regular public transport and no later than 2 hours prior to departure in international regular transport, in which case the passenger is not entitled to refund for bus station service, seat booking and boarding pass."
 
Tickets purchased online can also be returned via email at [email protected] (only for transactions from the Republic of Croatia – the bank account has to be Croatian). The procedure is carried out according to Article 4.2 of the Online Ticket Sale General Terms and Conditions.

Article 4.2 of the Online Ticket Sale General Terms and Conditions:
"If the customer does not want to return the ticket at Zagreb Bus Station offices, he/she may contact the Zagreb Bus Station staff, who will do so on the customer's behalf. In such a case, the request may be submitted in writing, via email only at [email protected].
The request may be submitted to this email address only during the working hours of the Customer Service, every day between 6:00 and 22:00. Outside of the Customer Service working hours, the ticket may be returned only in the manner described under item 4.1 herein.
The customer shall send the ticket or the identification code received upon successful payment (purchase confirmation) to the above email address.
In the event referred to under item 4.2, tickets shall be returned in the manner described in the General Terms and Conditions of Bus Station Operations, and Zagreb Bus Station shall, at the request of the customer who cancels his/her trip, refund the amount paid for the ticket, reduced by 10% of the ticket price for handling costs, with no possibility of refund for the bus station service, seat booking and boarding pass.