Same, both. It also squashes many "but but but we need you", shows that you're committed to it better than a verbal resignation.

Both. The reason for the letter is so that your SM has physical documentation stating your intended last day and so that if you're scheduled after that, you can point to the letter and tell them you are not obligated to come in since you gave notice of your terminating your relationship with Starbucks.

I gave them a letter stating the end date. Came in handy when my manager texted me later and was like “hey I need you to take so-and-sos shift this day” and I was like “sorry that’s after my end date” and she tried to argue with me, like, called me to yell at me on my day off any everything. It was ggrrreeeaaatttt

TLDR: make sure you have it in writing

As others have said, do both and make sure you either copy the letter or take a picture of it. If you want to be extra safe, you could also send an email if you have your SM’s email address. That way you have all bases covered.

I’m in the process of looking for a new job. I came home fuming yesterday and called off today. Best of luck to you!

I sent an email so that I would have record of me putting it in and a timestamp of when it was sent.

Do both

Always always always put stuff like this on paper. Generally speaking it’s polite to hand the letter to your manager in person, so you’ll give verbal confirmation as well

Both

You should always have your last day in writing for any job. If you don’t write out your last day they can go back and say you didn’t say when your last day was and guilt trip you into working. Write a letter and hand to your store manager.