Works out to £15 per kg. probably cheap compared to the cargo rate. A small shipment like this would incur the highest GCR, the minimum charge and likely assessed at a higher notional weight for low density cargo.
And unlike cargo you deliver the piece at one airport and pick it up at the next and you know it will likely travel on the same aircraft as you do. Big surcharges if you want cargo to move on specific flights and you still have to pick up & deliver to the cargo shed.
Excess baggage for small low density shipments is a bargain, compared to the other options.