As briefly mentioned you need to cycle a tank before adding fish- letting it stand for three days won't cut it, and it's pointless testing the water then as it will show you clear because...
Fish poop out ammonia, nitrifying bacteria turn that into nitrites, more nitrifying bacteria turn this into nitrates. Ammonia is very poisonous to fish in tiny quantities, nitrates are less poisonous (and makes fantastic fertiliser for aquatic and land plants). Periodic water changes keep the nitrate level low.
The water testing they do, (should) test for each of the three chemicals. If they are all showing up as zero I'm sure the shop will say, "yup, good to go" but it's bollocks, its uncycled water. A cycled tank will almost always have some nitrates, (the exception being incredibly densely planted tanks where the plants use up the nitrates as fertiliser as quickly as the fish can poop!) usually around 10-25ppm. As the tank cycles, the ammonia will rise then fall as the bacteria breed and multiply to deal with the ammonia. Then the nitrites will rise and fall, and then you will just be left with nitrates, which will continue to rise until you change some water.
To ethically cycle a tank you should use pure ammonia in an empty tank. If you can't get hold of that you can use fish food, but it is less exact and will take longer. Oh and by the way, once cycled and up and running, don't add large numbers of fish in one go- your tank will keep ticking over with the right amount of bacteria for the number of fish in it- double the ammonia and it takes a while for the bacteria to multiply- poisoned fish in the mean time...
I am not sure of the exact size they would need but have a look at cherry barbs. Very cute little fish, goldfish shaped, but tiny, bright red. Bog standard tropical community fish are easier to keep than goldfish.
So yes YABU in trying to keep a goldfish in an unicycle tiny tank... PAH are BU int hat they think a tank magically cycles itself in 3 days, and for selling starter kits at all.