One of my faaaaave topes! Undercover couple letters:

http://myrifique.livejournal.com/7395.html (Life, Community, Frozen)
http://lasskicker.dreamwidth.org/981.html (Psych, WWE, Lucha Underground, Person of Interest)
http://doyle.dreamwidth.org/478313.html (Arrested Development, Supergirl, Veep, Silicon Valley, Peep Show, Better off Ted)

Plus some general fake dating letters:

http://htbthomas.tumblr.com/post/136648524260/chocolate-box-letter-2015 (Crazy Ex-Girlfriend, iZombie, Star Wars)
http://ardentaislinn.tumblr.com/post/136383691056/chocolate-box-exchange-letter (Mad Max, Agents of Shield, Persuasion, Jupiter Ascending)
http://ijemanja.livejournal.com/128324.html (Crouching Tiger Hidden Dragon, Rosemary and Thyme, Calamity Jane, The Craft, Alien Resurrection)
http://randomuser28.dreamwidth.org/1909.html (ASOIAF, MCU, Star Wars, Orig fic, Les Mis, Harry Potter)