Go to the replacement page: https://myaccount.google.com/replacemykey
If you qualify for the return, there will be a box displaying the key you purchased (in my case it says "Titan Security Key Bundle"). If you do not see this box and you have multiple Google accounts, make sure you've selected the one in which you placed the order (and is paired to your account—thanks programd) by clicking on your avatar in the top right. If you're not simply in the wrong account, Google doesn't think you qualify.
At that point, you'll end up on the shopping page. Add the replacement key (it will tell you the full price of the item but don't worry). Proceed to checkout. On the final checkout screen, you should find a promo applied which brings your total down to $0. If you don't, you're probably buying another one so don't confirm.
I emailed them to the contact address on that web page, but no reply just yet.
I hope they have a return/replace workflow for unused keys because obviously, why would you want to use one before you get it replaced? Obviously.
And yes, I'm using the current Chrome and script/ad-blockers are disabled.