Been shopping at kinguin quite a lot since they often offer better price in spite of inaccurate pricing. With that said, I have not bought a single thing yet at price atvertised on gg.deals.
gg.deals always displays correct price before and after promo code (1st 2 figures in 1st screenshot), but the price after service fee never matches that I see at the checkout. At least not with conventional payment methods. This is EU region.
Here's 1 example:
I've tested this trying to figure out the service fee with steam cards a while back. I tried with 1 particular promo code and without it. If I remember correctly the service fee percentage was sliding between like 8-13% depending on how much you spend.
Ping me with a game name (or combination) and I can drop the screenshots and any other info