I have read that something like 5% of Starbucks' accounting liabilities are gift cards that have not yet been redeemed. So again in a simplified financial view, that is a loan at 0% interest, and some percentage of that "loan" will never get called in.
I guarantee you they have models about what percent and at what rate over time that "loan" actually gets "called in" (redeemed for coffee and store employees' time).