Remembering the anniversary.
To take advantage of free shipping, just make an order in a particular month, containing the listed game.
Here is a list of months and game titles, in which the promotion is valid:
February - "1831: For our freedom and yours - Battle of Grochow"
February - May - "Festung Breslau 1945"
April - "Bloody Fields of Mollwitz"
December - "Farmers vs. Empire"