I don’t know that there’s a solid legal argument. In one sense you’re working for Starbucks the whole time, in another sense you got an agreement to end your first shift early, and you agreed to pick up a shift at the other store, leaving the middle ground the same as a normal commute.

I think the one time I did something similar they had me put my travel time as time clocked in at the store in need- the original store has no reason to pay for it, but for a store that really needed help they can more easily justify extra time. That might be the right angle.

Can you call the DM? And explain?