Doesn't matter how "easy" you "think" it is. Let's stay on topic and HELP the op. He obviously doesn't mind spending $40. Perhaps the installer will walk him through it so he feels more comfortable next time?
Now onto your DIY list. I'll already assume that it's a poor installation based on the materials used or lack thereof. Perhaps he wants to do it correctly and wants an enclosure made as well as speaker baffles and whatever else it might take. Also, how will it look once it's been fabricated properly? It might take some time to make it look good if modifications were made to the door.
Personally, I expect a lot more than the materials you described. I'm not saying that your setup is terrible but it's not my preference but who's right/wrong? Did it meet your goals? Then it's perfect. Since it all comes back to preference and perspective; if the op wants to pay then let him pay. If he asks for installation help then let him ask.