I have imported Tropical and MArine fish for years as well as contributed to a major magazine in the field. The best policy is to avoid this method of purchasing fish. The prices are near wholesale but the shipping via overnight delivery like FEDEX usually wipes out any savings. and FED EX will not back any live fish deliveries so you are left only with the shipper to deal with. To ship airport to airport (the best method ) a minimum 100 # weight is usually specified and you have to go to the airport. Shipments from other countries require a Fish and Wildlife License.
keep in mind even good wholesalers loose fish in the first 48 hours after recieving shipments. Good wholesalers hold fish at least 24 hours before selling them. Even with an arrive alive guarntee you are still at risk of loosing your investment.
Sizes and condition of animals vary considerably so when using this method to obtain your Aquarium fish be cautious. Also see my guide on MArine Aquarium Fish about cianide collection practices.
There is however an exception to the rules when purchasing live animals online that makes the risk more acceptable. This is when a shipper has a proven track record for adequate shipping and he has a unique item. Under Unique items are the rare species not available elsewhere or at least locally. Also if the animals are captive bred you will have a much greater degree of success.