One ride, several addresses
Airport runs often are not point to point. Somebody has to be collected first, or a suitcase has to be dropped somewhere on the way.
List the stops when you book
The order matters as much as the addresses. Two stops across the city are a different drive from two stops on the same street, and the fare is worked out from the actual route rather than a flat guess.
How the waiting works
A short stop — a bag handed over, a key dropped off — is usually part of the ride. A long stop is a different arrangement and is agreed as such. Saying which one you need is the whole conversation.
The airport caution
For a flight, every stop costs minutes. If the pickup already starts at four in the morning, adding a collection point is fine; adding three is a plan that deserves a second look before booking.
Questions
Does a stop cost extra?
It can, and that is agreed when you book. A stop is part of the trip, so it is part of the fare conversation, not a surprise at the end.
Can we add a stop on the day?
Ask the driver. If the plan allows it, it is agreed there and then; if it does not, you will hear why.