Well while it's nice that you are cooler .. there are way more efficient ways of covering your windows to keep cool . I would imagine that while your 2 dollar role of foil reflects the sun it's still quite uncomfortable to stand near your window.
You might want to try a r-tech at home Depot for about 8 dollars you get a 4'x8' sheet of foam board covered in foil .. this actually keeps the inside area cool and reflects the sun as well .. cut to the shape of your window and your golden.. I keep these on hand every summer they go up and man does it make a difference . I know yours was only 2 bucks .. but this would be worth the investment and it's reusable.
Stay cool my friend
Home Depot use to sell a hundred foot by 4 ft. roll of an aluminum foil with mesh string in it. for $10 dollars You mount it on your rafters in the attic (staple) and it drops your temperature 10 degrees. I did the rafters over my garage and kitchen back in the 90s (there on the west side of the house) in TX. It helps a lot.
You're still going to get some heat pouring through that foil. You need to back it with a couple of layers of cardboard or something similar. I grew up in Florida and for a number of years I was poor and lived in what was basically a shack. This helped to keep me cool through a few summers.
I bet you got some strange looks, as you should put it on the inside not the outside. Also, get a thick reflective film that has a sticky side, not attach it with duct tape! LOL! I installed this in my bedroom windows like 10 years ago to totally block light (for better sleep) and lower the temp from the morning sun (also better sleep).
Best part is you totally can't tell from the street. It looks like any other window. Also, it just peals off the glass with zero mess.
That totally works as a temporary solution, but getting the adhesive off your window frame is going to be a problem. If it isn't very windy, I'd use a less tacky tape that leaves no residue.
The styrofoam board with aluminium on one side is definitely better but transportation of a 4x8' board may not be feasible for everyone.
It is absolutely not necessary to apply it to the inside and may in fact damage your window because of the heat buildup in the glass itself, since the sunlight has to pass it twice.
Stay cool everyone.