Which is why the improbable lunchbox doesn't really hit the bullshit detector as high as you might think, because she did leave a trail of clues for the Liars in the event that something happened to her. And if you think about the timeline of her last couple days on earth, something did happen to her. Like a wealth of things happened to her. A nonstop cavalcade of interesting meetings, kissing, videotaping, all kinds of things. All the things that could happen to a person, essentially, found time to happen to her that day; like, for example, she got killed at least three different ways I can think of. So all this planning wasn't exactly overkill, in the final analysis.
While the girls talk about finding a way to keep Emily in Rosewood until graduation, Spencer's been out buying a burner phone so they can text Ian anonymously and offer the footage in exchange for money. By asking for cash, they'll throw him off what is actually an insane quest for vengeance, and then he can show up and say something like, "Here is ten grand for those videos of you I've been taking since you were little girls that somehow prove I killed Alison one of the times she was killed," which Hanna -- being the most subtle Liar and the best under pressure -- will be secretly videotaping, and thus the filmer becomes the filmee, which solves exactly one of these girls' problems because there's still an infinite number of A's out there with sometimes conflicting agendas. Not to mention the other six people that probably also killed Alison that night.
Ian gets the text and goes stonefaced and they all feel kind of gross about how it's all coming true, even Spencer's like, "I've been so wrong so very often that I kind of got used to it," and then Mona comes running up to act all psycho and climb inside Hanna's sweater with her some more, so the girls all bounce immediately. Hanna leaves her phone, coincidentally (1), and Caleb immediately calls, coincidentally (2), so Lucas walks up, coincidentally (3), just in time to hear Mona hissing at Caleb something along the lines of, "Yeah, I read that letter and destroyed it while Hanna's back was turned, and I'd fucking do it again," of course. Whatever Lucas needs to put the story together on his own, that's what she says into the phone.