I don't know how the script is used so much, so I'm taking a blind concept shot here.
Achievements give scores right? Kind of like how puzzles give picarats in Professor Layton?
Well, according to the script features, the scores are stored in a variable.
You can make a 'shop' via events that handles the scores variable as your currency. It might take a bit of Conditional Forking, but hey, that's what eventing's all about, no?
You can also create a variable which stores the amount of gold you have, then switch the player's G amount temporarily to the amount in your scores variable, and after the evented store is finished, you can switch it back.