Sorry, but once again I have to state that I think that the soda-stamp deal onboard is a rip-off.
Me too - more so for adults. We get the stamp for our son since it makes it easier for him to get his own drinks, but I cannot recommend the soda stamp for adults.
I just did a test on our son's souvenir cup from the Liberty last year. With no ice it holds 20 ounces of liquid. I put crushed ice in and it took up about half the volume and I could only get a little over 8 ounces of water in the cup.