These nicotine doublers are for Australian customers only.
Doublers are designed to be diluted with a non-nicotine containing e-liquid. The doublers come in a range of different concentrations and are packaged with instructions for mixing, gloves and an empty syringe for dosing.
As concentrated nicotine is highly toxic, this is only offered to Australian customers in states which prohibit the sale of nicotine-containing eliquids. Check your state laws to ensure you are allowed to import nicotine before purchase.