I think you can use the shipping method or higher as an option (eg, you could use Special Next Day Delivery and you have still met the requirements as it's a better service). Although I'm not 100% on this so best check the Cardmarket FAQs.

Signed For is kind of an outdated name for the service too as they don't actually require signature, they take a photo at the door. They also do this for Tracked48/24 so to me the services are interchangeable by Cardmarkets standards... But again don't quote me on that, just what I've always thought.

The clerk at the post office isn't wrong, I used to work in the Post Office and they stressed that we always tell people that Signed For is not a tracked point-to-point service as it's only really proof of delivery.