When I saw the name of your game i new i have to play it and I really enjoyed playing it. It is sometimes really difficult and progression seems a little bit slow, maybe lowering the price by a little bit would be good. Like refills 8 and then 5 with the upgrade, upgrades 20 less.
A maximum day count would be cool too, like, you have x days until your soul will be sold, you can buy it for 300 and be free. This would also prevent cheesing the "Suprise me" and "Glasses" spirits. Right now you can just pass an empty cone to every spirit and get "infinite" money.
But overall this idea is so nice and I enjoyed it so much!