I offer to start your own business.
Be sure, you will be a monopolist in your region. So, you will be able to set prices on your own.
I offer you to create and sell disappearing and appearing inks.
Will not waste time and come to the point.
The Recipe of Disappearing Inks:
Mix 50 milliliter spirit based iodine and a tea-spoon of dextrin and filter sediment. The blue inks will discolor and disappear in 1-2 days because of vapor.
The Recipe of Appearing Inks:
Take lemon juice or milk and write. It will appear if you iron a paper where you write.