You can just let it ride and see what happens (as you say, it's only five bucks). You could also resend the invoice and give him a deadline of x-amount of days to contact you with his intentions.
But if and when you get to the point that you want to be done with it and relist the item you can file the Unpaid Item Dispute. Sometimes the UID will get that payment sent in. And if the buyer doesn't respond to the dispute then that's that.
Like you say, a money order could be on its way but you're within your rights to start that UID right now if you wanted to because he's already violated the terms of your listing.