If I pay extra for overnight shipping and you guarantee that my documents will get there at a particular time, they'd better be there at that time. I didn't pay extra for nothing. I don't care that it's the holiday season and that it's a busy time of year. You, as a business, should be aware of that and accommodate accordingly. This busy time doesn't change. It happens at this time every single year. If you can't deliver on your guarantee, then you should expect to refund my money.
I'm usually a very understanding person, but a guarantee is a guarantee. It's not my fault that you can't figure out the trends of your own business after 40 years.