In Serbia, you have two options: Pay the toll at every single tollgate by cash (accepted currencies: Serbian Dinar and Euro), credit card or purchase the on-board unit called ETC. With ETC, you have a discount depending on pre-paid amount of money.
The ETC on-board unit costs 2,022 RSD (approx. 17 EUR), that could be more convenient solution for you. You can buy it at authorized points of sales. To obtain at least 4% discount you need to charge it at least for 10,000 RSD (approx. 85 EUR).
If you drive frequently to Serbia (especially Belgrade), we recommend you to buy an ETC unit. It might be interesting also for those drivers that are looking for more comfortable ride even with post-paid method without discount (you pay only additional money for the on-board unit). Last option is to have your cash or credit card prepared at the tollgate. To evaluate your possibilities you can visit the official website: