If you have them packed in a sardine can first maybe. If you actually want them alive though, a medium flat-rate box will probably hold 4-6 fish, and a large maybe 10.
Thanks. That is true. The only problem with flat rate boxes is that they are not tall at all, and their is a 85-90% chance with goldfish that they would arrive dead because it would be hard to provide them with 2/3 oxygen in a 5" tall box. LOL!!!!