Let's start with the reason some US sellers don't ship to PO boxes.
Couriers like UPS and FedEx are widely used by US sellers because they are reasonably priced, always tracked and will pick up at home.
But -- since they require a signature from the addressee, PO officials cannot and will not sign for them.
Thus, no delivery to PO Boxes.
However, many sellers just copied the Terms of Service from other sellers, without thinking about them. So even those who use USPS for shipping may refuse PO box addresses.
Which is silly, because PO boxes are about the safest address the seller could get. You have to either have a key to the box or sign for your parcel at the counter, after all.
So.
You could send the above information to the seller and ask for a change of mind.
You could supply a street address. Your own (you don't live on a park bench after all). Your mother's . A friend's. Even at work if you know the guys in the Mail Room.
You could ask for a Mutual Cancellation. This won't work if the seller has opened an Unpaid Item Dispute, so act now before he does.