That's true. You should try to fatten up your fish before you mail it. I reccomend feeding it ham. Goldfish love ham.
What they love even more than ham is carp, they can't get enough of the stuff. Really any fish will do, but carp is the best you can feed a goldfish with.
However if you get lucky, the blood will color the goldfish, and you will gain the incredibly rare red goldfish. There is however a 99.99 chance of fatality.