I really don’t remember all the details of it. I never understood how Walt got it into the Stevie either, the show didn’t explain it clearly but a little poking around on the internet reveals a lot of peoplexare wondering the same thing. My assumption us he planted it on the table at the coffee shop before she got there. In this segment he comments on how “schedule oriented” she is so he probably knew where she’d sit.
https://youtu.be/00HXP9oBb9w?si=LUYdSn7Py_cw8uWO
Yeah, she must always have picked that table by the window if it was available. And there was only one package of Stevia for her to choose.