I have shipped fish for many years. I mostly now only ship rarely as I hate the work. But, how long fish can survive in transit depends on a lot of things. I live in New York about 35 miles NNE of NYC. Years ago I used the USPS to ship fish. I will no longer use them for this. However, Years back when I kept BN plecos they spawned like mad and I had to ship lots of them all over.
In early April one year I sent a box of BN to Alaska. For some reason it was delayed and arrived in just over 4 days in transit. All the fish were alive. A couple of years ago I sent a box of 20 zebra plecos to Texas via USPS. By then FedEX was moving most sucj packages for the USPS. An accident in a Chicago area facilty fropped a barrel of toxic chemical which broke open and anything in the facility stayes there for an additional 24 hours while they cleaned up.
I had packed the fish to arrive overnight and only used a 24 and a 40 hour heat pack. My mistake. The box took about 55 hours to arrive. Fortunately, I read all those research papers and one of the things I researched was acclimation. In particular, I read about what happens to fish if their water gets too hot or too cold. the researchers want to know two things. At what temperature will at least half the fish die and how do they behave as they approach fatal temperatures.
One such study also wanted to know if they saw the fish acting as if they were in the "death zone" could they be rescued. It turns out the answer was yes, but the way to do this was as quickly as possible to get the fish back into their normal temperature range. No acclimation was used, they moved them from near death directly into the right temp. This is how you save them.
So, when my "lost" box of zebras was located and they told us what time it would be delivered to my buyer, I contacted him and told him as soon as the box came he should immediately get the fish into warm water in the proper range. Incidentally, zebra plecos live warm. I keep them in the low to mid 80S F. To trigger them to spawn, keepers will often take the water temp. into the low 90s for a coupld of weeks and then drop it rapidly by 15+ degrees.
When the box of zebras arrived, there were 3 dead fish. My buyer took took the temperature of the water in all 3 bags and it was 59, 60 and 60F. The fish mostly survived. He did lose two more about 3 days later. Of course I refunded the cost of the 3 DOA. Because he had the rest for 3 days before the other two died, I sent him a refund for one, but he resfused to accept it. If he had tried to drip acclimate or even acclimate for 15 minutes, he would likely have lost a lot or all of the fish.
A good shipper knows how to pack. I now use more heat than needed to accommodate delays. Except for very young fish, I purge them by not feeding them for one to three days depending on their size. I add a small piece of Polyfilter to every bag as it will absorb ammonia and organics from the water. I do not lose many fish in transit and never have.
Perhaps the most important consideration when shipping fish is to hold the amount of water used as low as possible while knowing it will be sufficient to keep the fish alive for some time. The reason for this is when it comes to the cost of shipping fish, the fish do not really matter, it is the water that is being shipped. More water means the box needs to be bigger and will weigh more. Too little water means dead fish.
Finally, one must consider how many fish to pack in a bag. If there is more than a single fish in a bag and one dies, this can kill everything in the bag. If the fish are small, this may not happen. Also, the fewer fish per bag, the more water it will take and the cost to ship will be more. Here is the problem with this. I one can import common tetras for between 25 and 50 cents each depending on how many and their size. These fish normally sell for anywhere from about $2.50 to as much as $5 each. (I do not sell tetras but I do buy them for my display tanks.)
If a seller ships such fish they usually cram as many onto bags as feasible. if a few die, the lost to such a seller is small. If a seller shipped them 1/bag instead of one paying a couple of dollars a fish the cost from shipping will double or triple the cost per fish. There would be little reason to pay this much and the seller would never sell any fish.
The last time I bough any fish by going to a store, I picked up an 8+ inch clown loach. That was at least 5 years ago. I can say that, either in terms of numbers of fish or dollars spent for fish, 99% were for fish shipped to me. I have bough as direct imports, wholesale and from home based breeders/sellers. Rarely have I bought retail and had the store ship them. I have not bought on AquaBid since early 2006.
So to answer the original question, it depends. It depends on how healthy the fish are going into bags. It depends on how well packed they are. It depends on how long they are in transit. Finally, even if all the fish arrive alive, often trying to acclimate them will do more harm than good. All of the people I know who sell fresh water fish plop and drop. This means when the fish arrive, we open the bag and immediately put the fish into their Q tank. We do not float the bags to match temp. We do not drip acclimate. We do not do other methods folks often use to acclimate. We lose a lot fewer fish this way.
It is important to understand that there is a big difference between fish being shipped and therefore are in bags for close to a full day to as many as several days and the fish one buys retail and which are only in bags short term- from the store to home. There is another consideration here. When I sold fish at weekend events I always put more water in the bags than if I were shipping them. Since the buyer was carrying the fish home from the event, the weight of the water did not matter. This also applies when one buys from a shop in person. They can use more water and not have it raise the overall price of the transaction.