Not helpful I guess, but as has already been said, the valves should have been removed a packed securely and separately. I’m surprised that Royal Mail haven’t said that, or maybe they will.
Fragile stickers and tape are a great idea, I’m sure as much as possible the staff probably do take note, but a lot of the sorting process; especially for smaller items is automated, conveyer belts etc and scanners that read the bar coded labels.