Basically, it’s probably not possible to get enough Ricin into a single Stevia packet to ensure lethality in a 125 pound victim.

While staggeringly lethal when inhaled or injected — like, less than 2 milligrams for the average adult human — Ricin is WAY less toxic when eaten. You need 30-40 milligrams per kilogram, which works out for about 2 and a quarter grams.

Stevia packets are light; they contain only a gram of the sweetener. I can see doubling the weight and not tipping your mark, but slipping in more than 225% additional weight to a single packet strikes me as troublesome to the point of implausible.

Oh well.