Cancelling upgrade purchases

Hello,

today I purchased a couple of domestic UA flights in Y, and was offered to upgrade to C (or was it Z? Cabin says United First) for some fee right after I purchased the tickets.
I got the impression the fee UA asked for the upgrade was not too bad compared to the usual amount required to upgrade, and this offer was only available right upon the purchase of the tickets, so I bought the upgrades.

However, unfortunately something came up a few hours after I bought the tickets, and I need to cancel one of the flights now.
I was trying to cancel the flight and get the full refund on UA.com, as it has been less than 24 hours since I bought the ticket.
However, it seems I can't get the fee I paid for the upgrade to be refunded, at least online.
Are the upgrade fees non-refundable? (even within the 24 hr grace period?)
I can't find a definite answer to this on UA's website.
I am going to call UA and ask for a refund of both my ticket and the upgrade, but I just wanted to check here before I make the call.
Any help/advice would be greatly appreciated.
Thank you-