I posted this earlier.
The simple fact is that, UPS was backed up pretty bad from the winter storms in certain areas as well as a larger than projected number of packages sent. They had to clear that back log first. I know for a fact that UPS was hiring extra subcontractors along with more seasonal staff. Our UPS driver for the neighborhood showed up in a Uhaul truck last week and had mentioned the company rented extra trucks and even air cargo. He mentioned that just about everyone was working 10-12 hour shifts. I do feel for the drivers and warehouse workers, because of how hard they work during the holidays.
Now, the problem is the fact that UPS probably wasn't transparent enough with retailers like Amazon in regards to the backups. Amazon can only do so much, even with a guarantee. Its not like you are paying for 2 day or overnight and Amazon is sending it ground. What needs to happen is that the logistics side need to be improved on UPSs part, at least so it can be communicated to retailers and consumers. Those online timetables and guarantees are written way before the backups start and are usually estimates, If there were updates from Amazon and UPS when people order about backups in regions it would help greatly.
Also, I worked at a small retail shop before, now I'm sure Amazon is given more info than we were, but UPS would send us a general time table prior to the holiday of when stuff needs to be sent out by a certain time. We usually would offer free ground, once the timetable for ground ran out we switched to free 2 day and all the way up to free overnight to in theory get to you by Christmas. However UPS wouldn't send much else as far as backups, sometime we would have to call to check but thats about it. When you are dealing with hundreds of packages a day, its impossible to call and check if each package with be hit with a backup