This £3.50 charge upgrades the standard First Class signed for service that I use, to Royal Mail Special Delivery Service which is meant to be a Guaranteed next working day service (although in times of strike Royal Mail reserve the right to no longer guarantee all parcels to be delivered next day, but they have stated they are prioritising mail posted using this service.) Special Delivery is also scanned / tracked throughout it's journey, unlike the First Class Signed for service which is scanned when its handed to the Post Office, and only scanned again when delivery is attempted.