I will tell you something that probably most of members won't agree with me since I often receive packages that are lightly secured which proves that 90+% of the members pack watches that way.
This is as well senders fault, we are here speaking of 300+$ worth of goods that are shipped out in a small carton box without any additional external protection. I tape ALL of my boxes from outside with enough tape that it takes either 5 min for the receiver or any potential middle intruder to open the package. This discourages any postman, post officer to have their curious eyes and sticky fingers placed inside of the package. Of course it can happen but the probabilities are 0,0000001% while in this case the probabilities that somebody will have a look inside is high.
As I see this, it was a 15s job do open and take out the content of the package in your case. Trust me it would not happen if the package was properly secured/protected.
I had a similar case in the past year, lost almost 400€ of watches because the lazy fv%&r (sender/member on this forum) only closed the carton box and placed some scotch tape that could have been opened by barely looking at it for 3 seconds. When some post officers, postman see this they know there is an option to score, sometimes they do it, sometimes just by numerous handling the box falls apart and content gets lost.
Anyway, not something that should be done this way by the sender. They are responsible for our goods if they don't properly secure/pack the items we bought. I would suggest in this case a partial refund (in extreme cases even a full refund by the seller).