In a three week period I got vouchers from two different airlines. One you could use on line and one where the process is as Paul described. You can also take the voucher to an airport ticket counter and take care of business there (which I did, quite successfully.) Still sitting on $500 from the other airline.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016
Sail Windjammer: Mandalay 2017
Star Clippers: Royal Clipper 2015