Wait, if I'm understanding this right you should without a doubt be refunding $60 for the shipping. As far as the spoke he wants $40 and you are willing to give him $20. It's twenty F'ing dollars man, all this over twenty F'ing dollars. I'd expect both of you to be going, "No, my mistake for not mentioning it, I'll give you the extra $20." "That's ok, I should have looked more closely." "No take it." "No keep it." Shame on you both for not being able to resolve this.