You really think Walt slipped the ricin packet while he was coughing?? Are you serious?
It hasn't crossed your mind that he could place the ricin packet at the table before lydia even got there?
Yeah, Walt set it up BEFORE she even got there. That's why there was only ONE packet in the whole container. He mentioned how she likes to have the same routine and that's why he knew she'd be there. He met her EVERY Tuesday at the same time. She mentioned that she'll get the same order, which she obviously ALWAYS ordered there. He knew what her order was and knew she'd use Stevia.
Anyways, that's the whole reason why she asks for a SECOND packet of Stevia. The sweetness in the tea wasn't there b/c the first pack she put in was ricin.