The speaker is poking fun at his own spiritual life. He calls his Zen "thin" and a "bargain," as if enlightenment were something picked up on sale, and then admits he has "nothing to do with it." That line can be read two ways: he feels like a fraud, or he is saying that real Zen involves letting go of the self, so the less he is involved, the better.
The last two lines bring the poem down to earth. Turning over a sandwich is an ordinary, almost absent-minded act, and the remark that it won't last until supper suggests hunger, scarcity, or just the way everything gets used up. The title, "Running on emptiness," fits that idea: the speaker has little to hold on to, whether food or certainty.
The mood is dry, humble, and quietly funny. There is no grand insight here, only a shrug and a sandwich, and that plainness feels honest.