I agree that one has to pay attention to what box/container/envelope one picks up and uses when mailing items.
It's aggravating when things are moved around - I'm such a creature of habit that I sometimes forget to look closely when I'm picking up some item - wherever that might be - the grocery store, discount store, etc.
But is it really the post office's fault when I'm ignorant or careless?
I've brought home fat-free mayonnaise by mistake because I didn't really pay attention to what I was doing. I suppose I could blame the store for having that product next to the one I really wanted.