Welcome to the site.
First thing for you to do is go get
[*]
Saltwater test kits..ammonia, nitrite and nitrate (a master kit is cheaper)
[*]
1 big plastice trash can ...30g to 50g size. (mark it FISH ONLY)
Salt mix
A utility pump
Salt Mix + enough RO water to fill the can and churn it 24 hours before use. Match the SG of the fish tank.
Please don't think I'm being mean, I really want to help you to have a good experience in the hobby before you just give up and say you can't do it...because if you don't listen to what I'm telling you.... that's what is going to happen.
Since you stocked the tank without a cycle.... ALL of the critters are doomed to die unless you prepare to do water changes about everyday to soft cycle this tank. You are NEVER obligated to any store. You could order $1000.00 worth of stuff, they can't make you pick it up, or pay for it. So telling us you can't refuse to pick up a fish is not true, they are jerking you around. They may refuse to order something else later, but so what...do you have any idea how many online stores you can take your business to? They loose out if they won't order for you. YOU are the costomer and always right...remember that.
You don't need to just slow down, Al&Burke was being nice.... you need to stop buying live critters and purchase a book to help you understand a little bit before continuing. Live creatures eat and poop, and they poop in the same water they live in, without enough good bacteria to eat that poop.....it will poison the entire enviroment the creatures live in. Water changes the moment the ammonia begins to spike is the only hope that these creatures in your tank will have to survive.