(Reuters) - The main union at Royal Mail on Monday rejected an offer the company says is aimed at avoiding a strike that could affect mail deliveries ahead of a general election in December.Royal Mail told the Communications Workers Union (CWU) that if it removed the threat of strike action for the rest of 2019, the company would enter talks without preconditions. CWU General Secretary Dave Ward called Royal Mail's offer a "stunt" which the union would not fall for. "If Royal Mail are serious..