I’ve done it. I am not an electronics expert, but reasonably handy. You need the right tools and follow the instructions of e.g. IFixit closely. Buy a battery including the tools is safest. You’ll need a suction cup for the screen, pentalobes and a small + screwdriver. Plus a splodger to separate the screen from the case. A magnetic mat to keep the small screws separate helps. Oh, and reading glasses .
Cost me 20€ and about 30 minutes. End result: a working phone with a better battery (not original apple) and an owner annoyingly pleased with himself.
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Me too, and I agree with your conclusion.
You’re saving maybe $20-30 doing it yourself, but it’s a hassle because the tools you get with a battery in a DIY replacement kit are crappy. The ones the stores have are good, because they’re willing to invest in a tool that’s going to be used many times.
Plus there’s a non-zero chance that you lose or break something. If you lose one of those little screws, you don’t have a spare. The replacement place does.
Under no circumstances should you attempt to replace the camera in your iPhone. Those things are a bitch and a half. I spent an hour on it and ended up tearing the itty bitty ribbon cable right at the end when trying to put it back together 
i have done a 4s and a 6 battery and screen. Very easy.
I got parts from Amazon. ONLY get the parts that they stock, they fulfill the order.
i spread out a clean white linen napkin as a work surface, have VERY good lighting available.
There is NO glue holding the case shut but there are two glue/tape strips holding the battery in place. If you look you can find a tab to pull and stretch the tape out.
Good luck…Oh, the case screws are proprietary “pentalobes”. The battery screws are phillips.
Echoing the previous posters statement…the screws are VERY tiny. Do not loose one.