Perhaps Amazon could add a checkbox to the shopping cart: [] pay 10% more that will go directly into the pockets of the packers. I wouldn't even mind a picture gallery of everybody who handled my package.
Here's a discussion that covered the question in the context of allowing people to tip McDonald's cashiers directly, but the same analysis applies:
http://econlog.econlib.org/archives/2013/08/what_if_tipping_...
"Who benefits" is determined by the shape of the supply/demand curves, not who is being handed the dollars.