Buying a ticket for Valbiska to Merag
Jadrolinija sells this crossing at the port and through its own webshop. Both give you the same thing: a ticket for a date, not a place on a particular sailing.
Updated
You do not have to arrange anything before you drive to Valbiska. There is a ticket window at the port, you pay there, and you join the lane. Buying ahead is possible and it saves a stop, but it changes less than most travellers expect.
What the ticket covers
Jadrolinija sells a ticket for a date. The route page puts it plainly: a ticket bought for this line is valid for any departure on the date printed on it. There is no seat number, no vehicle slot and no booking reference tied to a sailing.
The webshop says the same thing, and it says it at the moment you are about to pay:
The purchased online ticket is valid for any voyage on the selected date, but does not guarantee boarding at the selected departure time.
The booking screen does ask you to pick a departure, shows you 10:45, 12:15, 13:45 and the rest, and then states that clicking one does not hold a place on it. You buy the crossing on that day, in whatever order the queue puts you.
Buying online
Both ports are in the webshop at shop.jadrolinija.hr. Choose Valbiska as the departure port and Merag as the arrival port, pick the date, and the sailings for that day appear with the crossing time next to each.
Registering an account is offered but not required; the flow continues without one.
Buying ahead helps in two situations. If you are travelling on a busy weekend the window at the port has its own queue, and paying in advance takes you straight to the lane. And if you are arriving late in the evening, you are not depending on the window still being staffed.
Buying at the port
The ticket window at Valbiska sits beside the marshalling area, so you can pay and join the lane in one movement. At Merag the arrangement is the same on a smaller scale.
Pay for the vehicle and for every person in it. The two are charged separately and add up, which is why a family in a car pays noticeably more than the car alone. Fares for every category are on the fares page.
Which one to choose
Neither option gets you on an earlier ferry. The queue does that, and the queue and boarding page sets out when to be there.
Buy online if you want to skip the window, if you are travelling in July or August, or if you like having the payment done. Buy at the port if you are not sure of your date, because a ticket tied to a date is worth less when your plans move.