How does the following paragraph relate to the novel's themes and postmodernism? …[I]f novelists truly wanted to simulate the delta of life’s possibilities, this is what they’d do. At the back of the book would be a set of sealed envelopes in various colours. Each would be clearly marked on the outside: Traditional Happy Ending; Traditional Unhappy Ending; Traditional Half-and-Half Ending; Deus ex Machina; Modernist Arbitrary Ending; End of the World Ending; Cliffhanger Ending; Dream Ending; Opaque Ending; Surrealist Ending; and so on. You would be allowed only one, and would have to destroy the envelopes you didn’t select. That’s what I call offering the reader a choice of endings; but you may find me quite unreasonably literal-minded. As for the hesitating narrator—look, I’m afraid you’ve run into one right now. It might be because I’m English. You’d guessed that, at least—that I’m English? I … I … Look at the seagull up there. I hadn’t spotted him before. Slipstreaming away, waiting for the bits of gristle from the sandwiches. Listen, I hope you won’t think this is rude, but I really must take a turn on the deck; it’s becoming quite stuffy in the bar here.

It can be argued that this passage encapsulates the postmodernist theme of many realities. Post-modernists question the dominance of the grand narrative. They argue that since the human experience is varied in terms of perspective, the grand narrative (an all-encompassing universal thesis) must give way to localized narratives that account for the diversity of perspectives.
In the above passage, the narrator Braithwaite insists that he is an accommodating storyteller and that he is open to letting the reader decide between a choice of endings. However, he maintains that the reader must decide on one choice from the selection he is given. As the reader, we are led to ask: Which is the right choice? The confusing array of choices Braithwaite presents epitomizes the nature of the post-modernist novel and, indeed, history itself. The passage seems to support the post-modernist argument that exact or empirical truth is elusive.
There are also many levels of narration in the novel: there is Braithwaite himself, then there is his adoption of Flaubert's persona as his own. Julian Barnes also weaves real historical characters (such as Jean Paul Sartre) into the narrative. As a reader, we are led to question the author (and also the narrator's) credibility. Indeed, the passage above alludes to this conflict that we have with Barnes's book: how can we tell what's real and what's not? Here, Braithwaite refers to himself as a "hesitating narrator," alluding to Barnes's use of metafiction (another post-modernist concept) in the book.
In metafiction, the narrator freely admits that what he's writing is fiction. Braithwaite's stream-of-consciousness discourse in this passage calls into question the linear, grand narrative inherent in traditional literature.
The passage above also encapsulates the intertextuality of post-modernist work. As there is no chronological sequence to Barnes's narrative, there is none in the passage above. Braithwaite's discourse follows no conceivable logic; first, he talks about a multiplicity of plot endings and then unassumingly mentions his failing as a narrator. Next, he's on to a seagull he's just spotted and his sudden need for a change in surroundings. In chapter 2, we are provided with three very different (and confusing) chronologies about Flaubert's life. The passage above (as well as chapter 2) support the post-modernist idea that the human experience (and perspective) involves many seemingly unrelated incidents or stories.
The passage above is a prime example of a vital post-modernist theme, that empirical or exact truth is elusive. Since each person perceives a particular experience differently from another, we must reject the grand narrative and embrace local narratives that take into account the individuality of each perspective.