I don't assume underemployment, I assume that employment isn't usually traded on a direct fungible-time-for-fungible-money basis (unless one is employed as some kind of freelancer). Most jobs come with an expectation of a long-term commitment, or at least constraints on when the work is done. It's well and good in theory to toss around the idea that people who are volunteering time to a charity could have just gotten second jobs and donated the money, but the odds that they could have gotten second jobs that would conveniently fill the empty time they had to offer - scattered piecemeal around their schedules - are negligible.

I don't think the abandon with which I purchase groceries is the same phenomenon as that kind of mental accounting, because I'm very conservative about non-food purchases in a similar price range, not just with major expenses.