By right all the service center use the same procedure, repair then pay upon collection of phone.
When you send the phone in, the engineer will check what's wrong with the phone. Then someone will call you from the RC to inform you what's the problem and the price to repair the phone. They will ask you for the green light for repair, if need to pay/out of warranty. If you agree to pay and proceed with the repair, then you will have to pay the cost upon collection of the phone. You might want to call in to the helpdesk to confirm on this before making a trip down to the repair center.
If is in warranty, normally they will just proceed to repair and let you know when the phone is ready for collection.