The offer is open to any Cloudflare customer. Cloudflare customers can claim this offer for Yubico Security Keys directly in the Cloudflare dashboard.
Yubico will ship the keys to customers directly. The specific security keys and prices for this offer are: Yubico Security Key NFC at $10 USD and the Yubico Security Key C NFC at $11.60 USD. Customers can purchase up to 10 keys.
I would recommend buying two keys, one for day to day use, one as a backup.
@gadget: No email from cloudflare or yubikey at all. But the offer page on cloudflare account says 'offer claimed'. I am on free tier basic account on cloud flare.