During sales this happens all the time. You have several bundles, but I cannot pay them all at once. I have to do a separate check out for each bundle, even if it is of one game (cause I have the rest). This is not just a matter of laziness (having to complete the process each time separately) but also a matt4er of cost since for each transaction I have to pay fees (paypal/credit card etc).

Just add all the games you want to your cart, then verify that the proper discounts are applied. Just did that with 5 bundles.

It might work, but still, going on a hunt to add each game then hope each discount has been applied, can't be the best way possible.

It feels weird that I am asking them to improve the way they take my money.

Open each promo to a new tab. Open each game you don't have to a new tab. Click "Add to cart" in each gamepage. Once out of gamepages, proceed to checkout. Recoil in horror at the price tag. Start removing games. Be satisfied with amount, buy them.
Still better than amazon, that only allows single item purchases from their digital games store.

And you honestly believe this is the right way of doing things instead of just having the shop/cart just add whatever I add from the sales page?

I don't know about Amazon digital store, never used it.

I don't claim it's the right or the wrong way. I just say that your "going on a hunt" is 2 clicks per game from the promo page. Middle click game name, click "add to cart". Sure, a "Add all games not owned to cart" would be easier, but it's not that much of a hassle as it currently is.

I think this is the first Christmas or Summer sale on GOG where I haven't had a single payment problem (like the process timing out, me not receiving any games, but my credit card being charged anyway. Quick email to support has obviously fixed those cases, but still.).

Maybe it is because unlike before, I am buying only few games at a time this time, not several dozen in one swoop. Or maybe GOG has just fixed something, or added server capacity.

For what OP asked... yeah they could make it less confusing if you do want to pay only once for lots of games, but knowing my previous experiences, I now choose to buy several times in smaller dozes instead. Then again, maybe with some banks or credit cards you save money if you are being charged fewer times? I have always the same charge regardless of how many times I use my credit card.

This would most probably be the most convenient solution. I like it very much.

I understand the label "Buy now" because that's the impulse that they want to set, but this should only add the bundle to the shopping cart whatever is there already. Simple and clean. An additional "continue shopping" link on the checkout page might clarify matters even more. It could discourage some customers though, especially if they don't know about it. It's not obvious.

Exactly, IMHO it reaches levels of being absurd to add something to your basket only to have it being replaced by the next thing you add to it.....on a sales day! Especially since when it is not a sale, it works by simply having things ADDED and not replaced!

I agree 100% that the current system is clunky. I would very much like to be able to directly add a bundle to my cart and continue shopping.

The method JMich posted works just fine, but it is an irritating waste of time and effort that shouldn't be necessary.