Groupon has RapidLash Eyelash-Enhancing Serum for only $24.99 – retail is $49.95! RapidLash nourishes sparse lashes and brows with a formula of vitamins and minerals. A daily swipe of the applicator across the lashes or brows infuses the serum into the hair. Within four to six weeks of regular use, brows and lashes appear lusher, and lashes develop the natural curl of a happy fern. One bottle of the dermatologist- and ophthalmologist-tested serum—made without harmful parabens or irritating fragrances—lasts about two months.