Lyric discussion by SweetT 

Cover art for Don't Pay The Ferryman lyrics by Chris De Burgh

"-why- a soul shouldn't pay right away"

Because, if the soul paid the Ferryman before getting to the other side of the river, he would swap places with him, and would have to ferry souls for eternity, or until he tricked someone else into paying him before reaching the other side. Which isn't really what you'd want to do, so basically, pay him after you've reached the other side. According to mythology anyway.

@SweetT I have read something different. From what I read, if you pay him before you get to the other side, he throws you overboard and you are stuck in limbo which is worse than Hades. The thing about the dancing bones in the water suggest that