Just tried it again now and it looks like they fixed it or it fixed itself. Thanks for testing things!
Since the beginning of the Spring Sale yesterday, MaGog has been seeing a fair amount of intermittent GOG failures and glitches, which is par for the course on all large GOG sales nowadays. Judging from past sales, it'll probably get back to normal about 1-2 weeks after the sale ends (i.e. in about a month).
That kinda sucks. I was going to buy a bunch of games but the actual sale price does not match what they're plastering all over the front page, actual game sale page, Twitter and other social media so I'd kind of expect they'd at least comment that someone is or will be investigating and fixing it. Otherwise it just looks like false advertising.
The games are non-essential so I don't really need to buy them, but I wonder how many people are being persuaded by the extra discounts for purchasing 3-5 games or more and then not actually getting those discounts. I'm sure they'll fix things eventually, but I think I'll hold off until after they fix them - if the deal still exists.