I tried to buy the discounted frozen cortex. I am unable to pay after I add it into cart and choose the payment option. I tried changing the payment methods. The pay for order button doesn't work.

Contact support and they will compensate you.They are usually unavailable on weekends but you still should be fixed up in time.

You can try logging in and out. The website is often buggy and maybe the checkout is not recognizing that you are logged in.

Also, are you trying to buy it as a gift? I believe GoG limits gifts for new accounts because of excessive credit card fraud. You likely cannot buy it as a gift.

You MUST buy this one FIRST:

https://www.gog.com/game/frozen_cortex

This is the main game, the other three items (Mega Tier, Ultimate Tier, OST) are DLCs ONLY and need the main program to be bought first or together with them.

You cannot buy "Frozen Cortex Ultimate Tier" or "Frozen Cortex Mega Tier" alone, they do not have the main game in those packages. GOG fucked up the DLC lineup here, I was wondering too at the beginning...