Rereading this gave me an idea: I think it would make sense to make the tonic quick to spoil at room temp. I think that since they’re free, requiring that it be kept frozen justifies why players can’t just carry a bunch with them wherever they go.
That also makes the putty more valuable, since it’s more portable. But unlike the tonic, it requires skill to use, so it makes healers valuable in a party. Since the tonic doesn’t require skill, I’d rather it not be so convenient that it makes Care & Medicine feel like useless skills.