I booked a fare that was labeled as refundable since my travel plans are not completely set in stone. However, when I read more about it on the Delta website, it say "fees may apply". What does this mean, if the ticket said refundable when I bought it?

Also, if I decide I want to upgrade my seat to something better and pay for that, but then end up refunding the trip later, will my seat upgrade also get refunded, or is that a sunk cost?

Thanks. Could not find the answer on the Delta website.