Is bus travel reliable in Croatia?

The bus is generally very reliable in Croatia for urban travel. Advance tickets are not usually needed. I can't say I would feel 100% confident for this specific itinerary, as sometimes things can go awry and it's not like another bus would be passing by momentarily. As long as you do not need to be in Zagreb for an immediate flight, it will probably work out fine.

We have taken Flixbus a number of places in Croatia, Germany, and France. The busses are modern. They are well-driven. They leave and arrive on time. The bus trip includes a smoke break about every 2 hours at a roadside restaurant-store complex, where you can use the facilities, buy a coffee-beer-wine, and get ready for the next segment. Flixbus is also economical - Zagreb-Zadar was about 40 kuna. For boarding, you can use the image of the ticket that you store on your phone. You don't need cell service - save a screenshot.

I found the Croatian buses generally reliable in 2015, though there was some confusion at Plitvice about where I should go to get the bus back to Zagreb. If you're staying at one of the hotels in the park, you should be able to get assistance on that score from the desk staff. They will probably have the schedule at their fingertips, too. My philosophy of bus travel, especially in the Balkans, is to believe only the schedule information I see posted on the wall at the departure bus station. Of course, when you're at a place like Plitvice National Park, there is no bus station, just a bus shelter that may or may not have some semblance of a schedule posted. This does lead to a degree of uncertainty.

I would encourage you not to plan on taking the last scheduled bus to Zagreb. That's not likely to be too limiting. I found the time allowance for the route I chose through the park (the second-longest of the mapped-out routes) to be overly generous even though I didn't get into the (packed) park until 11 AM and was definitely slowed down by the crowds. You can see a great deal of the park and still leave by mid-afternoon if not earlier.

I'm even more concerned (perhaps unreasonably through lack of knowledge) about your plan to depart Skradin on a "late bus" to Plitvice. Your park hotel at Plitvice is unlikely to be cheap; you don't want to be stuck paying the freight there if you don't even get to sleep in the bed.

I haven't ever bought a Croatian bus ticket on line; I move around fairly slowly and normally just wander over to the bus station a day or two ahead of time if I'm worried about a sold-out bus. However, I believe it's easy enough now to buy tickets online, though I don't know that all bus companies have that process set up. In 2015 there were at least two companies running buses between Zagreb and the park. One disadvantage of trying to buy all your tickets ahead of time is that you may be working from incomplete bus-schedule information.

Incidentally, you may be asked for a small fee (it was roughly $1 in 2015) for each piece of luggage you need to stow in the hold.

You'll find Wi-Fi available in a lot of places in Croatia, as elsewhere in Europe. I wouldn't expect any notification if a bus schedule changes, though I suppose it might happen. I'd expect to keep checking the appropriate bus companies' websites right up until the day of travel. And always, always pause when your arrive at a new destination to look for a posted schedule of outbound buses. Be sure you know how to interpret footnotes related to what day of the week buses run.

In order to use your US phone in Europe it must be: 1. UNLOCKED (not purchased from a major carrier such as ATT, Verizon, etc); and 2. Be capable of working with the European phone bands. Most of the newer GSM phones will work in the US and other countries. You can then purchase a sim card from the US carrier of your choice for use here. Then buy an account/sim card for the country you are traveling to. Swap the cards when in the other country. Remember to save your US sim card! Swap back when home. You cannot use your US phone number with the foreign account, but you will probably use email because of the time zone differences. European phone accounts generally have much more generous data limits for the cost.

Thank you very much to everyone that replied to my questions.

To Larry42: I have done the SIM swapping process maybe a dozen times and am aware of when and where I expect to have use of my US carrier phone number, etc. and the foreign carrier phone number, etc. My problem has been frequently that
the carrier associated with the SIM is not providing the type of service or time/number associated with the service
that they claim to be as noted on the packaging that the SIM came in or from other sources of the information. Possibly just my luck? My purchases have been at stores or kiosks by the carrier or convenience stores selling the same.

To acraven: From what you tell me I am skeptical of being able to establish a schedule that expects any bus to be at a
stop without allowing an extraordinary amount of slack time in my overall schedule to accomodate that. Particulary with the larger number of travellers in September I can't see that if you have not made reservations for everything well in advance there is a likelyhood that there won't be any availabilty.

From what I have recently read about Plitvice I would be interested in one of the longer "programs" which would take at least 4-6 hours. Given my interest in photography it would be more like six hours and not four.

There are very limited buses going from Skradin to anywhere other than major cities. I may skip going to Skradin completely. I may change my plan to go to and from Zagreb for Plitvice and arrive the day before and leave the day after my visit to the park, i.e. staying two nights at Plitvice. Yes sir, my hotel in Plitivice won't be cheap. Like you said, I don't want to spend a lot of money and not get to sleep there.

From a source I found on the internet I found a photo of a bus shelter near the overhead walkway in the hotel
area near entrance 2 of the Plitvice park on the northbound side of the highway.

One last question...has anyone heard of lugguge theft occuring? A website for one of the bus companies (I think) said that someone "accidentally" picking up the wrong luggage has occured with enough frequency to be concerned about. The website went as far as being on the lookout for suspicious people in the area and finding a seat on the bus above where your luggage was stored under the bus so you can watch it.

We've been to Europe 6 times in the last 10 years, and found little use for phones. We did get a SIM card on the last trip, but didn't use it.

What we use instead is Wifi service, which is ubiquitous. It's in most hotels, most cafes, and other places. We made most connections by email, which is easier to work with than phones anyway. We used the phone image of the ticket for Flixbus, not even printing it out. Much of Europe is at a par or ahead of the USA in terms of electronic and cell stuff. Croatia is a major tourist country, although it is not 1st world like France or Germany. It's very advanced in handling tourists. We had no issues getting bus tickets 1 day in advance for any portion of the trip. Most buses were not completely full, but of course that depends on time of year - September is not high season, and I don't think you will have any problems getting tickets. But Flixbus has a website. Go there and send them some of these questions.

It seems like things will be open in 2022. Will they be more crowded or less? Hard to say at this time.