On our first cruise on the Inspiration 2004 I didn't know you should mail the fun ship dollar certificates to Carnival by Certified Mail. I brought them up to the pursers desk later on the first night but they had to call Miami to confirm they were mine so I came back a few hours later and they were applied as on board credit. In 2005 we went on the Inspiration again and I mailed them certified as I did again in 2007 on the Sensation. I never had a problem with them and they were applied to my on board credit. I have started to purchase them for our April 2008 cruise on the Glory and I plan on mailing them again. I like buying them many months ahead of time and mail them all together so I have the copy of the hard certifcate as proof of purchase just in case. I use the fancy hard envelope as a tip envelope instead of the little ones they supply on board. Never, ever did I have a problem mailing them certified and never had a problem with them receiving them and applying them as a credit.