Before locking in your summer fare to Europe, see if you can’t find a better deal by searching an airline’s codeshare partners. Airlines use codeshares to sell seats on each other’s planes so they can reach destinations they wouldn’t otherwise serve. Since ticket prices constantly fluctuate, codeshare partners often quote different fares for the exact same flight. Inside, reader Christiana shares how she used codeshares to save almost $300 on a flight to England…