How do those states that use mail-in ballots deal with the delay that introduces? Must the ballots be post-marked in advance of the election to count? Or, do they defer calling elections until days after the election?
It varies. In Oregon, any ballot that is received by the post office by the cut-off date (8pm election day) is counted. In Washington, it's as you've described - any ballot postmarked by election day are counted, so it takes a few days to fully count everything.