I actually had to rewrite my TOS to include stuff like this.

Personally, I let people grab a slot on my commission list without paying, but I won't start until they DO pay. When they pay, I put a note next to their name indicating that they've paid and then start on their commission.

It's not fair to make paying customers wait for someone who hasn't paid yet. Surely both the nonpaying customer and the paying ones will understand that.