On partner hub search "pay verification" there should be a link to a service starbucks provides where it will generate employment or pay verification letters. You can also use it to print out every paycheck from every job you've ever worked at.

I’ve done one before for a fellow employee that was leaving and needed it for their new job. Your manager can provide one but there is no standard protocol with Starbucks. She should be able to Google a general format to follow and create one for you. If your manager won’t do it I would reach out to your DM. If it’s an informal situation a SSV could even make one so long as your manager is informed