A quick Google suggests in 2016 it was $1.17B, and earned $21M, or 1.79%.
I mean, once Starbucks have it, then the customers get it back via product (that has a margin included), or just leave it forever (free money!)
I have a firm "No vouchers" rule because of this, the vouchers in my part of the world inexplicably "expire" if not used within a certain amount of time, cannot be redeemed for cash, and will not be honoured if the business goes belly up
(¹I think this is too high; BLS thinks inflation over 2016 was 2.5%. But their core point still stands: interest earned was below inflation.)