On each game page there's a bundle if is applicable, usually they're made only with games of same series/setting. For example in any of the Heileen pages:
http://www.winterwolves.com/heileen3seamaidens.htm
http://www.winterwolves.com/heileen2.htm
http://www.winterwolves.com/heileen.htm
right on top of the page after the header, there's a text with:
"This game is part of the Heileen complete bundle! Buy all the games with 15% discount on total clicking the button below:"
I see. Experimented with games a bit. It seems, if you select 2 games of the same series, the discount is applied automatically, except Loren series and Bionic heart series (it must be because the first part has already been discounted, i guess).
