Thats cool......not. You shouldn't really be buying fish from Walmart in the first place. They are only in the fish business to make money, therefore all their fish are in bad conditions, under/overfed, and are most likely carrying a disease. Just my $.02.
The reason they charge for only one fish is to make up for the loss of fish you may have when you introduce them into your tank... naa just kidding.. Just make sure you quarenteen them first.