Four short, odd statements add up to a strange little story about writing down something trivial. The speaker takes the trouble to record, in messy handwriting, that a king wanted coffee, and asks that future generations know it. There's something comic in that: the grand word "posterity" attached to a hot drink.
The speaker also seems to know the whole effort is slightly absurd. They've heard "for certain" that the coffee will bring only "small pleasure," so the king's wish, and perhaps the act of recording it, is doomed to disappoint. The last line is the most puzzling. Giving away "a bottle of mist" suggests offering something almost weightless, and doing so "at a great loss" hints at sacrifice for little return.
The mood is dry, wistful, and quietly ironic. The poem seems to gently poke at how we preserve trivial things while the effort, and the pleasure, slip away like vapor.