I guess I'm a little curious as to why people are ordering masks for Halloween the day before?!? If I was going to do that, I would contact the seller to see if it would be guaranteed to reach me on time. And I would assume you would say you couldn't guarantee it, but the post office claims it will get there on time. Then I would be left to make the decision as to whether to risk it or not.
((Warning...small rant coming up...))
It's this whole mentality of I should be able to do anything I want any time I want in this country, and if things don't go exactly my way, well, somebody else is to blame. Angry indignation, coupled with an overwhelming sense of entitlement, seems to be the attitude I encounter most often these days. And I am sick of it!
((OK, thanks for letting me get that off my chest))
With the massive fires in California right now, I'm guessing that is causing rippling logjams in the postal system. It wouldn't surprise me AT ALL if the mail was taking longer than usual in California. When the anthrax mailings were happening, I expected my mail to take a lot longer, even if I sent something priority. It's just COMMON SENSE, which some people refuse to use.
So, you have offered a refund to these people, and they are still threatening you with bad feedback. What else is there that you can do? Unless, of course, you are God and have the ability to turn back the clock again a few days. And if you are God, I'd love to have coffee with you and wax philosophical for a few hours...
If you don't happen to be the Supreme Being, maybe you could hit them with the old 'What is it that you would like me to do?' Delivered with as much sincerity as you can muster up. Sometimes when you politely throw the ball into their court, they can figure out that there really isn't anything that you CAN do about this anymore, other than promptly refunding their money, which you have already offered.