help a buyer is claiming something is missing

[flowtter]

hi I need help a buyer  who brought from me is claiming I only sent them pieces of the the figure  sorry I think i posted in wrong area

the pictures they sent me of the figure sent to me are suspicious like it was tampered with in weird way. like they took the figure out and then closed it then sent a picture  claiming it was not there. its strange only the main figure is gone and all there is the little pieces still there. all the pieces seemed untouched or in the box in different places. they said usps never opened the shipping box.  the figure  I had described the figure condition in the selling description and it was factory sealed. I never opened it.  but it had that opening which you will see in pictures which i did tell them about but that opening was not large enough for it to come out and if it did the small pieces would be out too.  am in the process of trying to pic of the boxes pic of the sealed area which they claim is opened.  what should I do to protect my self. I have tracking and pictures and our conversations as prof. I know this could  to lead to a dispute. am ok with giving a refund if it was not of  my fault but USPS fault.  but in this case this seems scamish. i'ive never had issues selling and I have a good seller track record. this is my first time having problem like this.  how should I proceed with this. they have not asked for a refund yet but they will soon am guessing. 

here is the picture they sent me and they told me Hi. I received the package today but seems you sent the box without the actual nendoroid and just the pieces. Your ad was listed for the Nendoroid not just the pieces.  
Get back to me as soon as possible, thanks.

it wasn't in the box, at all. The nendo box wasn't sealed like shown the seal wasn't even on it at all. The figure itself wasn't in the box and the shipping box wasn't reopened by USPS. 

The plastic inside that holds the pieces is completely intact so there was no way for it to fall out of the front but I did check inside the box and it wasn't there. 

here how I sent it to them. i sent as the pictures are