Man, that's some horeshit. First of all, the mail had been discarded, right? You actually did that person a favor by forwarding them the mail. Not just that, but you included twenty bucks in the card!
I'd love to have the cops come to my house so that I could explain to them that I found a card on the ground, thought it nice to forward the mail to the person it was intended for, and even included 20 dollars.
I doubt any reasonable cop would write you up or arrest you for it.
The d-bag who threatened you deserves to be permanently bent over in a prison shower.