Argh! I went to buy Republic and ended up adding Expendable, Dark Fall and Advent Rising to my cart too!
Love ya, GOG, but the wife's gonna kill me ;).
It's probably best if you don't buy those games along with the promo as the promo price may not be applied. Buy the promo items in one purchase and the other games in another to ensure you get your promo discounts.
It's ok, I double-checked before confirming purchase and the discount was applied for Republic (already had Evil Genius in boxed form).
Thanks for the advice though :).