You are at the mercy of your buyer.
If he decides it was a present, there is little you can do.
More likely he will pay over the weekend and you will have worried for nothing.
If he has not paid within four days you can start an Unpaid Item Dispute. If he does not pay then, he gets a Strike and you get your fees back. Neither of you can leave feedback.
When the item arrives he might even pay for it.