Ignore and don't worry about it, you only got the love letter because someone mailed you something you didn't order anyway. Why would you respond to something that happened to you that you didn't know about? Clearly the postal service screwed up in sending something to you that was problematic for them, that was their fault anyway. Why would you do anything that could give you any sort of trouble in the mail? That would be foolish and you are a good person who never does anything wrong in your life. They are trying to nail a ghost for something that ghosts alone do.
As for overseas stuff, I would just avoid whatever return address would send such wanton things for a while and if it keeps happening, I would get a new postbox because obviously someone is trying to get you, an innocent individual, in trouble by sending things that cause problems. It was clearly a case of mistaken postage and you would only give yourself trouble by even interacting with it which is something that you wouldn't do because you only use the mail for legitimate means, like communicating with your family.
Hard to believe that someone would put such an onus on you. Sorry that happened, our mail services get things wrong all the time and certain forces try to make them create identities behind the mistakes that happen within those services. Gotta love how our agencies make these mistakes and then try to cover their asses by calling on the innocent to account. I always use PGP when mailing my friends anyway so that even if some asshole decides to question my innocence, they would have to do so much work just to even read my poetry I send to my girlfriend in China and I assure you, it's very bad and I would be horrifically embarrassed if someone did. She loves it though.