I'm going by the local Good Will and ReStore; one of the two had an analog postal scale a while back for $5; I'm going to see if they still have it.
It's a matter of principle to me; if a place loses my goodwill, they lose my patronage, regardless of what the place may be.
[Edit: I ended up using an old Toledo weighing scale for the packages. Apparently, if you ship a package through an actual post office, and not a contract station, you can still use handwritten customs forms and have them weigh the package(s) for you.]