My recollection is they do laundry by the piece not the bag. It can be expensive if you have a lot of stuff. They do have a special where you can cram all you want into a bag for $15 but that is usually one time during the week. Maybe its changed since then but I dont care. I am VIP on my next cruise and its gonna get done for free... yea for me!
2008-Valor and Glory
2009-Spirit and Splendor
2010-Freedom of the Seas and Mariner of the Seas
2012-Allure of the Seas and Navigator of the Seas