Totally understandable... from what I've seen, you should get your activation code after getting the shipping notice. You can contact support via https://support.google.com/stadia but they will likely tell you it may take 3-5 business days and to contact them again if you still haven't received the code by the end of the week.
The stadia's support via chat is said to be 'unavailble'. Only callback. WTF.
It's been more then 24 hours after my founder edition has been shipped. No code, obviously. No support. No communication, no updates.
Very sad stuff