mistermumbles: Actually, you can. You just have to add everything to your cart individually... which is a bit of a pain.
I have definitely made promo purchases that did not allow me to go and surf the site and add more things to my cart before making a checkout. Some promos immediately take you to the checkout page with no shopping cart icon appearing on the nav bar, and opening another web page to the site while you are looking at the cart does not show any cart with items in it. As such there have been days where I had to make 2 or 3 transactions one after another to purchase some games because there was no way to bundle them all into one single purchase. Other days promos you can just add items to the cart, shop around and add more, and eventually do your checkout. So the website's technology can definitely do it, and I must assume that when it does not let you do that - it is very much on purpose. The only thing I don't understand about it is what the rationale is - especially if GOG then pays an additional fee per purchase to a credit card company. There has to be some business reason for it, but it's opaque to the end user. :)
The only one thing I can think of, is that some of the promos move fast and possibly with limited quantities and they want people to make up their minds immediately and complete the purchase rather than locking up a virtual inventory slot for an excessive amount of time and another user missing out on the limited time or quantity sale, and then the original person not completing the transaction for whatever reason. What I usually do is add items to my cart, and if I'm not sure about any of them - I go watch trailers, read reviews, go on metacritic etc. and make up my mind, then I come back and edit my cart then purchase. That locks those games in my cart for the duration, whether it is 10 minutes or 10 hours or whatever timer GOG puts on it. I speculate that for some promos they need people to make their minds up and complete the transaction more quickly for reasons that are not clear outside of GOG.
All just speculation on my part though, it's never caused me any personal grief this way, but if it costs GOG a transaction fee then either their profit suffers, or their prices will be slightly higher to accomodate the extra fees one way or another, and reducing cost overhead can only be good for both parties in the end. :)