Why this varies
Most major airports charge a fee for cars dropping off or picking up passengers at the terminal, on top of any long-stay parking charges. Some airport transfer firms absorb this into their advertised fixed price, while others list it as an additional line on the invoice. Asking the question directly when you book avoids any surprise on the day.
Ask before you book
Browse airport transfer firms in Milton Keynes and confirm what is and is not included in the quoted price for your route.