To unlock the secret achievement shop, successfully complete three playthroughs of the game to complete the full story. Then, find the Strange Resistance Woman in the Resistance Camp. Talk to her to see a large list of options. Select the "Request unlocking you-know-what" option at the bottom of the list to display the achievements list. Each achievement is numbered, so you will have to check the description to view which achievement you are buying. The prices are 50,000 for easy difficulty achievements, 100,000 for medium difficulty achievements, and 200,000 for hard difficulty achievements. They are very expensive, and buying them all can be extremely costly, but this is an alternate method to getting all achievements.