Mine came after 24 hours of the shipping announcement (ordered 17th June).
When did you order? If you have ordered in June or July you should receive it soon, but you can always contact Stadia support as they can apparently send you an email code if you insist.
I can't tell you what's the difference, maybe they can see your code in the system and it would be the same that you would receive by just waiting. The code itself looks like "jhfdhflkdhfjhf458dkfjdlkfj," so as soon as you get it you can just type it into the Stadia App to save your name and set up your account.
Leaving feedback in the Stadia helpcenter is good, but you'll never get an answer to feedback unless Google needs to know more. Most of the times you don't get an answer though, but it's definetely reviewed.
The safest bet would be to contact Stadia support in your case.