I also ordered a couple of items on the 19th for pre-Christmas delivery, from memory the small print under the guaranteed Christmas delivery did very clearly state that you did need to use an option other than Super Saver delivery. Given that the 18th was the last posting date for Christmas (2nd class) and the 20th was the last posting date (1st class), ordering Super Saver on the 19th was very unlikely to work. Super Saver is not for time critical deliveries.
Anyway, commiserations on the rather public admission you cheap-skated the delivery on your wife's present! I hope she doesn't read the forum or think too hard about the reason her present was late.. ..