Offprints are unbound printed pages of an article, which a scholarly journal provides to the article’s author so that he may share them with colleagues. The protocol is — or rather, was — that when a researcher wanted to read an article that happened to appear in a journal he didn’t subscribe to, he would send a postcard to the author, care of his institutional address, asking for an offprint. And the author, as a matter of scholarly courtesy, would mail it to him free. My father is a scientist, and when I was little and collected stamps, most of them came from the postcards sent to him and the other scientists at his institution, requesting offprints. In those days, the 1970s and 1980s, the requests by and large came from developing countries, where the research institutions had less money for their libraries. The postcards came from all over the world, in other words, from countries I’d never heard of and imagined I would never see, and it gave me a thrill to see them, emblems of the glamour and global reach of the life of the mind.
He’s also offering to send you an offprint if you send him a postcard.