June-Sept is usually a lean season as schools and colleges reopen; carriers give discounts to keep their loads high No frills airlines have slashed fares in advance purchase offers to lure flyers and push up sales. SpiceJet is offering one way fares from upwards of Rs 511 and Rs 2111 (excluding taxes) on domestic routes and international routes respectively in a three day anniversary sale till Thursday while AirAsia is giving 50 per cent off on return flights booked till Wednesday. IndiGo too has dropped one way fares to little…