Pretty hard to maintain safety protocols using mass transit in a densely-populated city. A lot of those workers likely don't live in SF proper and have to get there by bus, train, whatever. And then asking those workers to schlep boxes of books to the P.O. every other day? It's just too risky and sorry, as much as we love books, these aren't essential supplies like food or toilet paper. I live in NYC and record/book stores had to do this too.