After reading this, something occurred to me - make sure that you haven't been opted in for "Guaranteed Delivery".

When it first came out a while ago, I opted out and everything was fine for ages, but then I recently had a buyer open a case on the day an item was onboard for delivery and I had to wait several days for resolution. I did discover, to my horror, that Ebay had quietly opted me back in to Guaranteed Delivery which had allowed the buyer had opened the case, even though I had sent with tracking and within my handling time! 

So, double check that you are opted and out and perhaps extend your handling time by a couple of days.