The booking agency for Surfer Blood thinks there's a second show on June 1: http://windishagency.com/artists/surfer_blood
They would probably officially announce it when this one on May 31 sells out.
2011: Sold out show at Showbox at The Market (capacity 1,100). 2013: Fanbase increases 50%. Tour manager logic: let's book them at The Neptune (capacity 800).