Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
No-one here can provide proof the product key was not used, this is just a user to user forum, if you have scratched off the key card to reveal the product key, most likely Walmart will not accept the keycard back for a refund.
You could try contacting Microsoft Support directly, but they also may not be able to provide proof the product key is unused, try using this method to contact their support.
Go to this link: https://support.microsoft.com/contactus
Select Microsoft 365
Click 'Get Home Support'.
In the box provided, type billing.
Press Enter, then on the next page, scroll down and click 'Sign in to Contact Support'.
Navigate to Microsoft 365 and Office - Manage my Subscription.
Select the 'Chat with support agent in web browser' option