Ok, suppose we have to start with as many details as poss please about your tank.......anything to do with it please help....as much infor would be helpful, and possible pictures too. Fish/plants/substrate/invertebrates, 'normal' levels of ph, gh, kh, ammonia, nitrites and trates.
Reading your post, from the sounds of it, you do not acclimate your fish to the water? Floating is used to balance out the temp, but thats all it does. Fish can go into shock if taken from one water source....lets be extreme.....with a Ph of 3 in the bag, and put into the tank at 9. Also the differing levels of am, trites and trates can have the same effect. The best way to deal with this is what is commonly referred to as the 'drip method.' Basically after letting the bag float in the tank for approx 15 mins for temp acclimation, roll down the sides of the bag so it floats open side up - i peg mine to the side of the tank too - and set up a bit of air line tubing with a knot tied into it so that there is a slow drip of water into the bag. Other options, and what i prefer as its easier for me, is just to slowly add little bits of water from the tank to the bag every 5/10 mins. I start off very small, say, 50ml, and slowly increase the amount that goes in by, say, another 50ml each 10mins.
I would also think about what you do after you add the fish. Moving for them can be traumatic and the best thing we can do to help them keep calm during this time is keep them in the dark. Hence why your LFS no doubt puts the fish bag in a brown paper bag, or something similar. This goes for the tank too. Before you start floating them.......turn the lights in your tank off and leave them off. For the Whole day. And there is no need to feed them. Leave them alone and let them 'find their feet' that first day stress free. A nice quiet environment would also help.
As for the reasons for their deaths so far, with current info, we can only speculate and from what we have, id say stress/shock may be a factor in it.
As per keeping goldfish in a bowl that you mentioned, well my opinion on that piece of animal cruelty isnt for the eyes of the children that might read this