I highly doubt the reasoning that just because patients were feeling better without a CURRENTLY KNOWN medical explanation, therefore it
must be a placebo effect.
I, like a lot of other people here, have done my fair share of alternative treatments. One of them even involved daily injections of heparin. I stopped when I noticed it wasn't making a difference. In that case I still have a needle stick that would presumably give a similar cortisol rush, but I was able to determine it wasn't actually helping me feel better.
I find consuming large amounts of salt as very important for my POTS. I wrote another post about it
here. By the time I measured how much sodium I was consuming by simply listening to my body, it was much less than previously, but it was still
over 5 grams/day. According to the FDA I should cap it at a max of 2.3g per day. My blood pressure was low and my sodium levels had just tested out as normal.
Back when I was severe, I needed to drink several glasses of unflavored electrolyte water (with something
like this) per day, in addition to salting my food liberally, multiple times throughout a meal. I did this for years, and with gradual improvement of my overall health it naturally tapered off when my usual electrolyte water tasted too horribly salty by the third glass, and then eventually by the second glass, and finally down to nothing.
A friend of mine with similar issues was prescribed salt pills by her doctor, because she couldn't bring herself to eat as much salt as her body needed to counteract the orthostatic problems.
I wonder if for some people, the salt is the important factor to IV therapy? It's cheap to try, anyway.
As an aside: I grew up knowing how to regulate my electrolytes by taste because as a kid living in Cameroon we often had to walk for hours or sometimes even days on steep terrain in tropical heat. Replenishing electrolytes lost by sweat was absolutely essential. Each person carried two water bottles - one was plain water, the other was water with salt, salt substitute (potassium chloride), and sugar. This is a basic oral rehydration solution (ORS), and can also be used to replenish electrolytes for severe diarrhea.
This stuff tastes absolutely revolting if you don't need it, and delicious when you DO need it.
At least, that's always been my experience. We were instructed to drink plain water until we felt thirst for something else or a bit lightheaded, and if so try drinking the ORS. ORS brought immediate relief, and we'd keep drinking it until it began to taste disgusting, then switch back to water.
Another trick my mom taught me is sprinkle a little table salt directly on the tongue. If it tastes good, your body needs it. If it tastes bad, spit it out and forget it.
My dad tells the story about how when he climbed Mt. Rainier, he started feeling very ill, so bad he was doubting whether he could continue on at all. The leader of the group gave him a couple salt pills, and they immediately had an amazing revitalizing effect, to good as new.
Most people I talk to these days are very hesitant to let themselves eat as much salt as they want, because of all the info out there about how it's "bad for you" to eat a lot of salt. But everything I have ever seen and experienced firsthand and heard secondhand has been along the lines of what my mom taught me: your body knows how much salt it needs, just let your tastebuds guide you. This is why I will never drink flavored electrolytes, I can't rely on the 'yuck factor' to stop me when my body has had enough! It's also another reason junk food is bad for you, they mask the salt taste so you tend to get more than your body needs without knowing it.
Oh, and because I KNOW someone's going to mention "but there are people who need to be on a low-sodium diet" - maybe so, but my grandpa was one of those people, and he followed doctor's orders to the letter regarding the consumption of sodium, and it landed him in the hospital in serious condition because his sodium levels were far too low. If I hear "everyone should eat less salt" one more time I swear I'll rip my hair out.
Tl;DR: If you're interested in trying saline IV, try salt or unflavored electrolyte water. It might help.
I have troubles with salt tablets causing constipation and really bad postural hypotension, but I do love the taste of salt. I think it made a difference when I had berroca with table salt added yesterday. Fiance made a weird face and said it will taste gross, but I quite like it. Going to look at the drink you linked. I've tried gastrolyte and it seriously is horrible. 
