Almost every commission dispute comes down to something that was never written down. These are the clauses worth having in place before you take a single payment.
Payment timing
State when payment is due and what happens if it is late. Forty-eight hours is standard, and it needs a consequence attached.
Revisions
Say how many rounds are included and at which stage. Sketch-stage revisions are cheap; changes after colour are not.
Usage rights
Be explicit about what the buyer may do with the finished piece and what you keep. Commercial use should be a separate conversation with a separate price.