I have a few hundred of these self-adhesive Japanese stamps on that are still on paper. My usual technique of dunking them into water to get the stamps to peel off is not going to work with these.
The proper method is to spray the back of the envelope paper with isopropyl alcohol. The liquid soaks through the paper in about 30 to 60 seconds, dissolves the acrylic adhesive, and then the stamp can carefully be peeled off.
