Postal question

I sold a dragon on eBay last week that is floating around Pittsburgh, it was there and should have been delivered on Monday. They just popped up with it’s undeliverable. I contacted the buyer to confirm the address; the address from Paypal is correct except I got the last digit of the ZIP wrong. After the irritation- I mean how dumb can you be just look up the street address- I got worried that they will decide to keep the package and not return it to me. I will be going to my post office tomorrow to get answers, and I do have the receipt from the mailing. I am just concerned they won’t either deliver it to the buyer or return it to me so I can resend (hello UPS). Anyone have an other ideas on what to do about it?

Edit- I bought insurance and tracking; the ‘undeliverable’ said they may try to return it to sender if possible. I did write my return address on the side of the box; it’s not on the label they spit out and if they can’t figure out street addresses maybe the can’t read marker on the side of the box either.