So I am likely to buy a used Glock 19. I've heard I can have a used Glock refurbished for free -- I just need to pay shipping (~$60?), no questions asked. Is this true? Does this mean I can buy any G19, and then ship it off to Glock to have it rebuilt like-new? If so, that'd be sweet
Pretty much. You only have to pay them money if you want the slide refinished. Most of the time it isn't necessary.
If you have an FFL you are on good terms with, you can pay them to ship it and it will cost you less money because they can priority mail it. (only FFLs can send handguns through priority mail).
-Mike