buy requested a return

[pandazbr]

I shipped 2 boxes of books to a buyer (27 books), tracking numbers showed they were both delivered today. Buyer contacted me and said they did not receive all the books and now want to return. I asked them if there was damage to the boxes, because they claim that they only recieved 11 of the 27 books I sent, and I don't know how 16 books can just disappear. They claim 1 box only contained 1 paperback. The weight of the the packages was 12pounds and 13pounds...as stated on the USPS receipet. 

What are my options? I feel like I'm being scammed here. 

They submitted a "There's something wrong with the item your buyer's received, so they'd like to return it" 

They said, as part of their reasonsing, that  "no notification from shipper or usps on packages"  Which I don't understand, since there are shipping numbers, which I have supplied. 

[pandazbr]

Thanks for the replies. They did contact me before filing a return. I contacted them first to give them updated shipping info, since I could not fit all the books in one box and shipped 2 boxes. I included the tracking information. They responded saying that they received that packages, but they did not receive all the books. I let them know the weight of the packages, and that when they left the facility, they were full packages and all books were accounted for (I wish I would have taken a picture before sealing the boxes, but hindsight and all that). I reached out to ebay and submitted the BBL and return abuse. I have no way to know if they are telling the truth or not. I'm just a very, very casual seller. I should have also added insurance, but I did not, so the USPS won't let me file a claim online. I will try calling them tomorrow when they are open. I have until June 9th to respond to the buyer. I'm probably just going to have to take the loss, unfortunately.