String Custom Field Radiolist Buttons Not Appearing

[Hawker]

I'm selling prints which can be unsigned, or signed for an extra $5.

As these don't need separate child products, I've made a String custom field called 'Print Signing'. There I set 'As radiolist' to 'Yes'.  When attaching this custom field to a particular product, I've added variants called:

• Unsigned (extra cost = $0)
• Signed (extra cost = $5)

When I save this and check VM3's checkout page, I see

Print Signing Unsigned
Print Signing Signed

...but this should appear as a choice radiolist list. There is no way for customers to indicate which option they want? Can anyone tell me why the radiolist buttons are not appearing?

[Studio 42]

DO you have set this as a cart attribute option in the custom field ?

[Hawker]

Oui (yes)