From the sales announcement:
It is now the dead of Winter, ice covering the land under the polar vortex as the bitter cold forces people back into their caves. To celebrate staying warm and playing a few games, we are having a Winter Sale of 20% off for all orders, even for the smallest of items. This discount can be applied to an order in the shopping cart by entering the coupon code "amg" and hitting the "update" button. Existing special bundle deals are excluded from the sale as they are already discounted, but those bundles can be mixed with other regular items in the cart and those items will be discounted at 20%.