If the P+P is high and the packaging wasn't good enough you should be able to say so and not feel threatened. It's a bit much if you can't be honest as it makes a mockery of the feedback system and it might just as well not be there. What would happen if it had been damaged in the post because they hadn't packed it properly the post office wouldn't accept responsibility for the loss, it would be down to the sender, and that would end up costing them money. In fact your comment should actually be seen as helpful.
Jan