It's not a cheap 'quick fix' which is why many body shops won't touch it.
It can be done without replacing the panel but it requires time to do it right (meaning more money and more time before you get your car back).
I had a rust repair once at one body shop with a 'lifetime' gaurantee... but that stubborn rust spot kept coming back. After they fixed it the 4th time for free, they told me they were very sorry but no more 'fixes' and that I would have to get a new door to fix it for good because they did the best they could do (sliding door on a caravan).
Instead the caravan got sold being replaced by a brand new one.
Rust (if caught early enough) and fixed properly the 1st time CAN be fixed but it's not easy to do