ebay calculates the estimated shipping times with no allowance for holiday season volume. and some buyers use that as a real-world guaranteed delivery date.
"opening a case" has been replaced with "make a request". the buyer was just "requesting" some reassurance (as stated in their note). they can't do this until the estimated arrival date has come and gone. I would suggest you increase your handling time (adds to the estimated delivery date) as much as you can without throwing yourself into a higher fee schedule, or get used to more "requests" - I think buyers expect every seller to provide service like
Amazon (and maybe don't realize that some of us are 1 man/small family operations) and the new request feature is going to be used a lot more in the future.