Whenever I go through a toll bridge, I frequently pay the toll of the person behind me. I've sometimes wondered if it propagates backward at all. For the Starbucks card, I would love to live in a society where this could work. Unfortunately it's one of those things where one anonymous bad egg will ruin (i.e., drain the account) for everyone else. Some way to restrict it to a community with less anonymity would help (…
In practical terms, paying the toll of the person behind you may not directly propagate to tolls being paid. However, the recipients are going to be just that much happier or less stressed out, and are likely going to pass that mood along to the people they interact with in some way.
It may only make a small, localized part of the world a tiny fraction of a percent better, but it adds up.
I think it was Steven Covey who wrote somewhere about the trim tab on a giant ship's rudder. It's expensive/hard to build a motor that can turn the rudders on a giant container ship directly while it's in motion, but all it takes is a little mini-flap on the edge of the giant rudder.