Reserve or book tickets?

In an app I am writing the user can book/reserve tickets for riding a bus.
Which of the following terms does fit this process best?
'Reserve Tickets' or 'Book Tickets'

Also, in some cases the user would need to make a change/transfer, but can buy tickets for both buses together/at the same time. How would one best describe this process?

Typically, "reserving" tickets means that you haven't paid for them yet, while "booking" tickets means that you have. A reserved ticket can be given away if you don't come in and pay for it by the deadline, but a booked ticket is guaranteed to be yours because you've already purchased it.

In the UK, people normally book tickets for different things.