I suspect that the reason for the law favoring the recipient is to avoid companies running a scam where they send stuff then try to charge for it or pursue the recipient for not paying for return shipping. This is a "good thing". (been a victim of such an attempted scam myself -- company sent me a bunch of garbage and tried to charge me $400 for it).
I usually draw the line at where it would be worthwhile for me to go back and claim the difference if I were overcharged. If I'm overcharged a few dollars and already made it home, I let it go. So... if I'm undercharged a few dollars, I'll do the same. When it gets to be ~$2000, I'd most definitely be spending quite a bit of time trying to get my money back, so it's only fair that I spend some time trying to give it back.