To be more clear, are these included:
– opinion pieces, and
– letters to the editor? (I assume yes and no, respectively.)

@yaboi69 correct. Opinion pieces count, letters to the editor don't

I'm unfamiliar with the NYT. What's the difference between the two?

Letters to the editor are written by members of the public and chosen by the paper and appear in a section named "letters to the editor"

Thanks. Are opinion pieces not also written by members of the public and chosen by the paper to publish? Or are they written by the newspaper itself?

@AndrewBrown Usually by staff or, 'guests'