I would send one last note with a deadline in which you'd like the commission completed. Give them a couple of months just to give them breathing room and to allow for life to happen should it do so. But tell them that they need to respond and let you know if that will work and if not an EXACT timeline in which it will be done, otherwise you want a refund. I'm pretty sure you're outside of the refund status on paypal, but send them one last note with this. If the deadline comes and goes, whether it's the one they agree to or set, then i'd post a beware.