...and several hours later I still have:
A 'mystery game' ...
I should try buying another one to see if checkout is still borked for me as well..
I'm pretty much in this same boat. But I'm concerned that getting another game will just get you another Mystery game at this point.
At the beginning of the sale there were only 13 of the listed games I didn't have(On GOG, I have some of these elsewhere):
A Golden Wake
Agarest: Generations of War
Airline Tycoon Deluxe
Breach & Clear
Breach & Clear: Deadline
Cities in Motion
Gabriel Knight 20th Anniversary
Shantae: Risky's Revenge - Director's Cut
Supreme League of Patriots Season Pass
The first mystery game I got was A Golden Wake leaving 12 I didn't have.
And then it gave me the mystery game bug.
Since it prioritizes games you don't have I'm worried I'll just get another mystery game since I don't think the period has reset for a new "supply" but there are definitely some games there I want. Gabriel Knight 20th Anniversary and then Airline Tycoon being the most wanted by me. (Agarest and Eminage gothic are the only ones I don't think I care for).