I recently bought the game through Steam and 500 AC, but the AC isn't showing up when I try to buy a new character slot. I haven't played PSO2 on any other platform until getting it on Steam, so I'm not sure what the problem is. Most people seem to have issues with Xbox/Steam getting mixed up or something, but I've only ever just now played it on Steam. Is there any fix to this?

It's only 5 bucks, so I'm not super stressed, but it'd be nice to know if I should just avoid making any purchases from now on.

Check your purchase history on Steam to see if it went through. If it did, try logging into a character and see if the AC shows up in-game on that character. If it does, you should then be able to go back to the main menu and buy the character.