Being more accurate is always a good thing, right? Well, maybe not quite so much. See, the original developer post for World of Warcraft explaining how flight will work in Shadowlands included the note that once players had cleared the requisite quest step necessary to unlock flight, they could purchase the Expert riding skill needed to fly and start soaring through the skies. Unfortunately, the wording could also be read as meaning that players would have another purchase once they had completed that particular step.
As a result, Blizzard realized the slight lack of clarity in the wording and changed the official blog to make it clear that no additional purchases will be necessary. Once you unlock flying, that’s all you need. If you need a little more evidence, though, the developers also reached out to Wowhead with a specific statement:
The “to purchase” wording in the Shadowlands Flying blog was only there to indicate that the player will need to have trained Expert Riding. The vast majority of active players have had that for ages, the wording was just included for the sake of accuracy as Expert Riding is not automatically granted upon unlocking Shadowlands Flying.
So there you go. No, you won’t have to buy a thing to fly in Shadowlands, outside of the things you’d need to buy to fly anywhere. Simple!