Overasked straightening question

Hey guys and gals, I'm sure this question has been asked many a times,but It'd be greatly appreciated if you could answer it one more time :)

I'm a 16 year old guy with hair about 7.5 inches long (when straightened), and right after showers (well, a couple hours after showers) after my hair dries, it ends up curling to around like 3, 4 maybe 5 inches. It's really annoying, it flairs up and about with no real control, so I've been using a flat iron for about the past 5 months or so. I like the results, but there are some days it turns out horrible. So I'm wondering, is there a thing I can use to just throw it in there during a shower and after it dries it will have pretty-much the same effect as using the iron?
I've heard about relaxers but it sounds expensive and complicated
I really don't want to have to spend too much, over $100 is really too much for me to spend on just my hair haha, so yeah any working/cheaper/sexyfied means of doing this would be greatly appreciated :D

u dont need to spend 100 dollars just buy a good relaxer from the drug store probably a mild one and get the help of a friend who has done that stuff. going to the salon is better, i would say thhat u cant make ure hair straighter with just products you need to heat style but ure hair isnt too curly so its best u relax it u probably wont need to do much to it after that and i doubt u will have to keep up with relaxing it because ure hair isnt super curly.