Never-mind, I think this explains it (from IMDB.com):
"Regarding Lydia and the ricin explanation: Last week, they ended the scene of Lydia's meeting with Todd in this same restaurant with her asking the wait staff for Stevia. Walt had met Lydia at this place himself in the past so that's how he knew she had weekly meetings with "the cook", and also knew that Stevia was her sweetener of choice. Also, whether she met Todd or Walt, she always sat at the EXACT same table. When the scene began in the final episode tonight, you can see that Walt is already there... but is sitting at another table. That means he was there first so had time to put the ricin in a Stevia packet before arriving and then put it in the container with the assortment of sweeteners that was on the table she always sat at. Also, there was only ONE pack of Stevia in there when she reached for it."
"Regarding Lydia and the ricin explanation: Last week, they ended the scene of Lydia's meeting with Todd in this same restaurant with her asking the wait staff for Stevia. Walt had met Lydia at this place himself in the past so that's how he knew she had weekly meetings with "the cook", and also knew that Stevia was her sweetener of choice. Also, whether she met Todd or Walt, she always sat at the EXACT same table. When the scene began in the final episode tonight, you can see that Walt is already there... but is sitting at another table. That means he was there first so had time to put the ricin in a Stevia packet before arriving and then put it in the container with the assortment of sweeteners that was on the table she always sat at. Also, there was only ONE pack of Stevia in there when she reached for it."