For the bus from Zagreb to Plitvice, while it is possible to pay extra to reserve a seat, the bus driver may not be aware that you paid extra and may re-seat you to accommodate his friends. You can ask a local for help.
The bus stop at Plitvice, entrance 1, no longer sells bus tickets. Also, the driver does not sell tickets. You can buy your bus ticket online through the Flixbus App. The bus station website may not work. The driver seemed to allow one middle-aged couple to take the bus to the next stop to buy their tickets at the station. Be aware: people will tell you that you can buy bus tickets at the bus stop or from the driver. This is not true. As I was waiting to get on the bus, a lot of passengers were provided this misinformation, didn't have tickets and were scrambling to figure out how to acquire a ticket.
Taking the bus is a pain in the ass.
Zagreb tram tickets: Unless you speak fluent Croatian, you may want to use Google translate to have a written translation of what tickets you want to buy from the ticket seller. The ticket sellers may not be able to understand English or foreigners attempting to speak Croatian.
Taking a domestic train in Slovenia: Slovenia now charges supplement fees when you buy an open ticket for a domestic route and take an international train. If you know you are taking an international train ahead of time, you can pay at the train station. If you hop on an international train with your open ticket without pre-paying the supplement fee, you need to pay the conductor in cash only. I paid EUR 1.50.
Getting from LJU to Ljubljana at night: You may not want to consider booking a shuttle through a taxi company. You'll get mobbed by taxi drivers in the arrivals area and you won't be able to find your driver amid the commotion. The driver is upset because he can't find you. You are upset. Everyone is upset. Go-Opti now offers a shuttle service for late-night arrivals for EUR 11 (they used to charge more than the taxis).
Sarajevo airport bus: Vijećnica bus stop
https://maps.app.goo.gl/iV3RtDnX8bDWnuGw5
You can catch the 200E bus to the airport from here. There's a bus shelter, but no bus stop sign, so this stop is easy to miss for visitors. The reception at my hotel helped confirm this location. It is at the north-east tip of city hall. The bus stop for the 200E further east by the kiosk may be less convenient if you need to carry your luggage further. Also, the bus stop further east may be congested with tour buses and tour groups, so the driver may not see you. At the Viječnica bus stop, it's less congested. Also, if there's a problem with the 200E, you can catch the 31E here to Dobrinja and walk a little bit to the airport, so you have a backup plan. Taking the bus to the airport was surprisingly painless and easy. It only took 25 minutes, and there were only two passengers including me. I thought it would be super crowded like the Budapest airport buses.
Sarajevo trams: You can pay with paper bills and the driver will make change for you. The only way to buy a tram ticket at the train station is from the driver.
Bosnia cash: A lot of places are cash only, including museums. Hotels may charge a 5% fee for paying by credit card. Be sure to exchange enough cash with the expectation that most places will not accept cards. You can exchange Euros for BAM at an official fixed rate. Banks will charge a 1% service fee, but you'll receive a proper receipt. UniCredit has kiosks in English and tellers that speak English. They are very tourist-friendly. (Be sure to bring your passport or photo ID.) While you can pay with Euros in many places, outside of Mostar they may charge a 10% premium (e.g. the Gazi Husrev-beg Mosque). I missed out on some yummy restaurants because I didn't have enough cash, but I guess that's just as well because lots of people smoke inside restaurants.