Thanks PromZA for working that out. I've been sitting here scratching my head at why the total amount was wrong, and wondering if GOG had introduced regional discounts.
It never occurred to me that the additional discount would be added to only part of the full price. -_-
I suppose next we'll be getting "50% off - buy them all for another 50% off" sales for games. <sigh>
Meh, it's not that bad. Discounts are never taken in total, they are always individual. If a store has an item on sale, 50% off and I have a discount code or coupon for 15% off, it would never be 15% off the original price, but off the sale price. The sale price is the new full price. Even when I worked for a retail company we had an employee discount of 10% and buying on the credit card would eventually get you (or anyone) an 10% off card. They are always done separately. $100 - 10% ($10) = $90..then $90 - 10% ($9) = $81. Never get 20% all at once. Especially these days and online. That's just not how computers work.
At most the release should say "...an additonal 15% off the sale price..." to be more specific. You get half off each item. If you happen to buy all three then it registers that fact and deducts the 15% from that new total.
That being said...<$4. I'll take a fly at that. From what I saw online it looks like it might be interesting.
The games are great value at 57% off, and I'm certainly not criticizing that. I only noticed the lower than expected percentage because they were an insta-buy for me at this price.
But all the examples you give are of independent discounts, so of course they're multiplicative. Everyone would expect them to be. By contrast, sales in the format "X% off plus an additional Y% off for doing something" are often additive if both discounts are part of the same "deal". For example, my electricity company words its 40% off plan as "30% off plus an extra 10% off for paying on time".
Having read a million or so GOG sales posts, I know that the usual wording is "buy them together for an additional Y% series discount", which doesn't generate comments about discount confusion (presumably since it's clear the discounts are independent, not additive). I know I've never attempted to add a series discount to a launch discount, but the way this sale was phrased caused me to do just that. Hopefully GOG will return to the previous, clearer wording for the future.