If you've had your iMac's hinge repaired by Apple, you might be entitled to a refund.
Apple will apparently start refunding customers who had previously paid for repairs to the display hinge of their iMac. An issue affecting iMacs shipped between December 2012 and July 2014 could cause the display to stop adjusting properly and tilt forward, with repairs often costing around $100.
This information comes from an apparent internal service document, obtained by MacRumors:
Unlike some of Apple's other Exchange and Repair Extension Programs listed on its website, the details of this program have not been made publicly available. Apple has instead sent internal communication to Apple Authorized Service Providers with information about repairs and refunds.
Since Apple hasn't made details of this program public, you might want to try contacting Apple Support for help if your iMac is among the eligible models.