What is P.S I love you based off of?
According to the How I Met Your Mother wikia, and confirmed by listening to both songs a few times, "P.S. I Love You" bears a striking resemblance to "You Oughta Know" by Alanis Morissette, most notably in the verses. Certainly, Cobie Smulders' vocal performance brings Morissette to mind. The intros to each song are quite similar, with just the singer and an almost identical drum beat. Even 20 seconds in to each song, the rest of the instruments kick in, with a notable bass line in each.
I should note that the breakdown that occurs at about 1:30 in "P.S. I Love You" doesn't mirror "You Oughta Know" at all. However, the acoustic guitar in it vaguely reminds me of the acoustic guitar in the chorus of Morissette's "Ironic". I don't know if that's just a coincidence, or it's just me.
Either way, the song definitely feels like a nod to/parody of Alanis Morissette, and bears some similarities to "You Oughta Know".