I was lucky that my first encounter with this problem was a white car, a minivan. White is much easier to match with off-the-shelf “rattle can” spray paint. I was able to do a passable job on the hood though I didn’t get the flow quite right and the texture is a bit rough to the touch, and doesn’t shine as much as it should, but it’s pretty good. That paint is 100% intact six years later, even though this car never sees a garage. I later did the entire long roof, and was pleased with the results - not perfect but MUCH better than the rusty mess it had become. The only challenge is deciding what to do at the edge where the new paint meets good existing paint. No easy solution. But if you do your trunk lid, you can do the entire thing and avoid that problem.
Colors other than white are more difficult. I was surprised to discover that the local automotive paint store has the ability to put their automotive grade mixed paint into rattle cans. It’s more expensive than spray cans off the shelf, but you might get by for $50 - $75, mostly because you have to buy a minimum amount of mixed paint. That way, you might be able to DIY on this job: sand and mask it, get the surface really clean, prime with off the shelf primer, then paint. The auto paint store will probably give you all the help and instruction you need. It might be best to practice your technique on some scrap metal with a cheap can from a hardware store before you head for the car. Almost anything you do will be an improvement. As mentioned above, the prep work is the biggest part of the job, especially the sanding. It’s not skill, just time and effort. Don’t skip that part.
One other idea: the local auto salvage yard referred me to a guy who does auto painting as a side job. I haven’t talked with him yet, but the implication is that I might get him to shoot the roof and hood of another car for maybe $200. So you might ask around.