Caffeine works as a diuretic to reduce puffy eyes, and as a vasoconstrictive to reduce the size of blood vessels. 

Purpose: Reduces puffiness and inflammation, reduce size of blood vessels. Reduces fluid retention around the eyes leading to reduction in eye bags, increase eleasticity and moisturization, and decreases appearance of dark circles in 14 days. 

Suggested usage: It will depend on the version you buy. The one I have has a suggested usage rate of 0.5% to 5% in the cool down phase. 1% to 10% in the cool down phase below 40˚C (104˚F)

Solubility: Water soluble

Shelf life: About a year, could be more. 

Cost: 30 ml or 1 fluid ounce is $5.25 Canadian, so 17.5¢ per ml (Windy Point Soap Making Supplies)

You can buy many different versions of caffeine, some powdered, some liquid. The one I have is a water soluble liquid – INCI:  Aqua (and) Glycerin (and) Coffea Canephora Seed Extract, Sodium Benzonate, Potassium Sorbate, Citric Acid – with a suggested usage rate of 0.5% to 5%. I use it in the cool down phase as I don’t have enough information to know how it tolerates heat. In the formulas in this e-book, this is the version I’m using.