The same process happens in metro areas. Collection centres dominate and are staffed by phlebotomists not cashiers. When a collection centre is in a clinic it is often a separate business to the clinic.
In my personal experience whenever there has been a charge it was usually collected at time of booking on the phone when a booking is required, or the bill arrives in the post some days later that could be after the sample is taken, whether or not a booking was required.
