Hi, IamDandu. Sorry for the confusion, but thanks for reaching out.
I've initiated a refund request with itch.io (they handle all the payment stuff - it's one of the cool things about itch for small indie creators like me!). This is the first time I've had to issue a refund, so I'm not sure what the turnaround time is on that or what to expect, but assuming there aren't any issues, you should get the refund soon and then can go back and buy each of the individual volumes separately to get the bonus content.
Note that you didn't need to pay $15 to get the bonus content – you only need to pay $3 for each one, for a total of $12. I'm of course appreciative of tips on top of that, but if you want all the booklets including the bonus content, it should only set you back $12. Buy each volume individually at $3 (or more) each and you should be good to go.
Again, sorry for the confusion, and I hope you enjoy the game.