Sorry, but neither of your examples are correct.
"Less" is a comparative term, so you could say that one of your boxes contains "less than the others", or "less than it should".
"Short" refers to the amount that you are missing, not the amount you have. Your box should contain 200 items, but it only contains 185, so it is 15 items short. Idiomatically, you can just say that the box "is short".
I think "short" works better in this context. You could say any of the following (not an exhaustive list):
These use "less" if you really want to, but they don't sound quite as natural:
I will send back the box that has less in than it should.
I will send back the box that contains less than it should.