They probably have a good model for what percentage of those will never be redeemed so they wouldn't have to count the whole $2 billion as a liability. The OP's one big customer would be harder to predict the future behavior of.
I’m aware of actuarial science, but what does that have to do with accounting? (We've been talking about liabilities as an accounting and contractual term, not as a remedy for injuries.)
Neither of these show up either on a balance sheet or cash flow statement. If a contingent liability is probable, you have to record it as a liability per GAAP.
Oh. Then what's the effect of the growing pile of unredeemed non-expiring gift vouchers that companies issue? Is there a little asterisk next to liabilities saying "but don't worry, we're sure we'll never have to pay this"?
