Vonnegut is referring to two things in his typical acerbic way. For one thing, Americans often think that "things" will buy them happiness, and frequently these "things" are useless pretty junk.
But more importantly, I believe Vonnegut is criticizing the platitudes found on gift shop trinkets, philosophically dumbed down phrases that have little or nothing to do with thinking. For example, "Grow where you're planted!" or "If you love something, set it free. If it comes back to you, it's yours!" ... things like that.
By purchasing these items with their words of dubious wisdom, we try to give the appearance of caring and thought, and fork over the $9.99, and hope it works.