In terminal, the desk is inside the terminal.
On Airport, in the facilities of the airport, normally at the car parking.
Shuttle bus is a free service that some suppliers “on airport" offer to their customers so they do not need to walk to the office/counter.
But, in the case of Krabi, as it is a very small airport, there is very little difference between a supplier that is located “in the terminal" and another that have its location "on the airport".