Shipping bag chemistry is a very complicate issue. Most deaths are not from ammonia but hyperoxic and hypercapnic conditions. I explained this here long ago. It is covered in great detail in Spotte, 1979, Seawater Systems: The Captive Environment.
Having the water hyperoxic, like most bags, just causes a dysfunctions the Bohr and Root Effect . These two physiological functions detemine how much O2 is stored and released. A fishes respiration rate is a function of the DO in water. The more there is the slower the rate is and usaully dependant on nothing else. As the CO2 increases in the bag there is a shift in the paCO2 and pwCO2. Meaning that in time the fish can not remove the CO2 from its blood which ends up turing acidic. The fish "calls" for more O2 in its blood physiology, which it gets, due to the Bohr and Root Effect. However, the fishes gills see that the O2 in the water is high, so the respiration rate does not pick up and now the fishes blood is low in O2, low pH and high in CO2. So, the Bohr and Root Effect have ended up releasing, prematurally, all the O2. The fish now die from blood acidoses, due to hypercapnic and hypoxia condition, brought about by hyperoxic water.
Wheter or not a fish dies in a bag, from floating or not floaing, is call casued by diffusion rates of CO2 and what happens to its blood chemistry. Floating a bag can kill a fish just as fast as not floating it. If the bag is very high in CO2 it will diffuses out ot the bag faster when in water, at the junction of the bag water interface. This can cause a rapid shift in pH. Opening the bag in this case, in air/non-floating, cause less of a shift in the pH.
The above is all from shipping fish and not form the LFS to your house. When it comes to LFS fish it is another issue. It s not EVER, IMHO, the ammonia or CO2, it is the salinity and more so the temperature. This is where the acclimation comes from, it is for Salinity and Temperature. If the salinity and temperature of the bag are the same , DUMP fish in tank
To warm of tank water and cool bag water will make the fish some what hyperactiv, if just dumped ie. Warm bag to cool tank is a No-No, it is much more stressful and the fish often just "crash" to the bottom. To high a Salinity will cause the fishg to over mucate and its "skin" can't "breathe" right. Warm water to cold water is about the worst thing you can do to a new fish.