A dealer is going to sell you the manufacturers recommended coolant, the same on that came in your vehicle originally, so you can’t go wrong. If you are still under warranty, the dealer should bring the coolant level up to optimum free gratis. They may do that for you even if the warranty is over. It will only take a cup or two of coolant to reach the max when hot.
However, you do not have a problem. Your coolant level is just fine as long as the coolant level in the reservoir does not go below the low line when hot.
Your cooling system has about 6-8 quarts of coolant in it. You can add a cup or two of distilled water to the reservoir and it will not have a significant effect on the overall performance of the system. The premix protects down to 34 degrees below zero, one cup of distilled water probably wont change that by more than a degree or two.
If you are just not comfortable with that, or just leaving it where it is, you may find that the dealer may actually sell the coolant below Amazon’s price. You can call them to see. That would be a smart business decision.
I don’t think it is an issue to use a universal long life coolant, especially for just a top off. They sell for about half the cost of a manufacturer specific coolant, but if your not comfortable with that, OK. Your money, your vehicle, your responsibility and you have to sleep with your decision. And its less than the cost of a tank of gas so cost isn’t that much of a factor when you put it in that perspective.