Thought it was important to share our learnings related to booking bus tickets from Vienna to Prague. We foolishly waited to try to book bus tickets once we got to Vienna. Long story short, we had trouble processing our US credit card on both Westbus and Student Agency websites, as well as PayPal.
As an alternative, set a note to Studemt Agency asking if we could pay cash. You can do this; however, it is a gamble as to whether there will be available seats on the bus. At the time of our inquiry, there were 10 seats available so we decided to go for it.
We arrived at the Praterstern station in hopes that we could find open seats. As it turned out the bus was delayed by ~1 hour. The Praterstern station does not have a kiosk or information desk so there was zero visibility into the delay (which was not ideal as it was cold and raining outside).
Once the bus arrived, we quickly found that there were no seats available and the 10 open seats were filled at the first stop.
I will say Student Agency was very quick to respond to our inquiries and the bus looked modern and nice.
We ended up opting for the train as our credit card worked and it was a sure bet that we would get tickets to our destination.
Only writing this post in hopes that i can share our experience to save others valuable travel time and trouble. Book your bus tickets in advance!