People send stuff back because they put too much carrying volume on their bikes.
Maybe, but it hard to imagine that folks base their list on what fits in their panniers. I know that is not a factor for me. I plan the packing list and gather the gear before I pack it. I don't add stuff because my panniers are only half full, but maybe some do.
I think the problem is the mind set of taking "x" might be handy and "y" sounds like a good idea. A better mindset is "can I get along without any of the stuff I am carrying or can I replace anything with something lighter".
I find that as time goes on I carry less and less and my panniers get emptier. I strive to find a few items to leave home or replace with lighter items each trip. At some point I will hit the minimum I am comfortable with, but I am not there yet. I have a feeling that about 20 pounds including the panniers themselves might be a sweet spot for me, but I am not there yet, maybe next trip or the one after.