Well i been shipping fish for years now and been ship/received thousands of fish over the year. I believed i have had 3 DOAs on me caused of leaked, other than that, everything perfect.
I also have shipped large flowerhorns many times. how about 15"

yup one single fish with water, bags and box weight more than 20 lbs.
If the fish still has any sign of life, I guarantee it will live, most of sellers prefer ship fish in cold season gather then hot. That's why heatpack made for

. I have shipped flowerhorn to area cold as 18F with one 40Hrs headpad and the fish are fine. The key are good insolated box, unfeed the fish at least 2-3 days(I make sure the tomach empty, craps = Ammonia), with bag buddy and 100% pure oxygen to help them calm.
Bags/water: Doesn't matter much if the fish 6", i will use the bad of 8". As long the fish could straight it body and water just enough to cover the top fin or a little bit over. main is oxygen.
Shippers try to make fish less jumping around in bag as much as possible, because the fish will get stress and build up ammonia, pin the bags or damage itself.
One thing that most receivers mostly miss out when things happen that only blame the shipper but forgot about the carriers. Shipper do not know how the carriers will do with the box. could be kicking, throwing since they need to meet the deadline. So even if shippers wrote(Handle with care) on the box, well that's no use.
In your case, if your still still have any since of life. Best just fill 50% of water in your tank and water in the bag in a bucket or small tank. water just fill up more then 10" for stressed fish. add one table spoon of salt per 5gals of water with strong air stone to provide oxygen then the fish will slowly recover. water temperature also is another issue. You must pay attention and make sure just few degrees different between water in tank and the bag when you mix together. and slowly increase it up above 80F.
It's will kill your fish fast if water in bad are 68-70F and you throw the fish in 80 some degrees water. the fish will get shock and die soon or later.
But am sorry for your lost.
Most people always blame on seller when received stressed fish when shortly die after that. but sometimes it also on the receiver's know how to acclimate the fish too. but it's best always contact with your seller about the fish condition so she/he know and work with you if the fish not make it.
As for refund, it's up on you and the seller. Most sellers are strickly on their policy, most sellers will work with you to resolve the problems for better in both side, and a few sellers will likely take a lost to increase their reputation and of course also a few sellers being *** try not to refund the money back.
But normally. if the fish DOA, both seller/buyer will take the lost, buyer will loose the shipping money and seller loose the fish. After that is up to buyer pick out another replacement fish or get your money back.
hope that's help