It is heartbreaking to get a neg especially when it's not your fault:-(  I would do as one of the other posters has suggested - stop offering Express Post and use Click &Send instead.  One thing I never ever do is guarantee delivery as AP can let you down on delivery times.

It's worth a try to get rid of your neg by phoning ebay, and keep all messages with your second buyer through ebay's system.  Threats of negative feedback have a good chance of being removed as ebay can read the messages:-)