If you feel bad about returning the chemicals, just discard them. You can do that later (anytime), just give these guys a chance to show you the way. You'll see it's worth it in the end. I have a pool store near me that uses the same products. I still get my liquid chlorine from them and I got some stabilizer from them. It's okay to support a local business, but trust these folks. You'll be happy.
I would definitely look into getting my chlorine from the pool store. I get a 12.5% chlorine from mine. Also, I would not worry so much about adding liquid chlorine. When these guys tell you that you need chlorine right away, dump gallon or two in there. It's not going to hurt a thing to go over the recommend level. I had my FC up to 25 last week. A lot of people recommend a weekly shock. It's not a big deal. Get some chlorine in your pool.
As for Muriatic Acid to lower PH. I was very nervous about using it. Do some reading on usage and storage. It is however, very effective and very inexpensive. Learn/read about putting the gallon jug right into the pool water and pouring the acid out with the lip of the bottle just a little bit above the water level. It is so much easier to do it that way. But, MA is wicked stuff so be careful. The thing is that the powder they gave you is pretty nasty too. It's acid. And, the powder makes it seem less nasty - it's not. That powder gets on stuff and causes damage.
Get your chlorine level up first. Follow the directions you've been given and you'll be fine in no time. MA can be learned in time. Your post said you're at 7.8. You may not need to adjust it for awhile. I just got my pool last year and I found this site after about a month of ownership. In that time, I've been vary happy with my pool. I've had to work through some issues and learn a few things but there's never been a time my water wasn't clear. That includes when I opened it after it's first winter close.