UPDATE
All three of the lost postcards have arrived.
The last one took 130 days to travel 337 miles to Germany. Postcards to Germany usually only take a week.
It does make you wonder where these postcards were, especially the last one that went missing for over four months.
But I’m happy that they all finally arrived. Now to see if all those other expired postcards suddenly arrive.
No mystery.
They hung out with each other in the back room of the post office after finding an abandoned case of Merlot! Naturally, it took them a long time to get through it. Then, they had to sleep the party off. Then, they had to dust themselves off before making their way into the postal system.
Of course, each one had to stop along the way to their destination to share about their party adventures with other postcards.
But, be sure, none revealed the location of their private party. 
It seems so obvious now!
Wow! 16 cards expired, that is far too many. I hope they start arriving for you soon.