Live sphagnum moss in a small cup. This is plenty to top dress one plant or two sparingly. You can also use it to start your own living sphagnum moss culture.
Please note that sphagnum moss requires high humidity to stay alive or it will dry out and die. To use as a top dressing for a plant, you will need to grow the plant in an area with high humidity. You can also culture it in a closed container or use fogging or misting if the humidity is low.
The living sphagnum you receive will be fine in its plastic cup till you need to use it and will even grow in it for as long as space allows. When you receive the cup, you can simply place it in a bright area (without direct sunlight) and it should remain healthy for a long time till you use it.