$400 ain't bad for that lil' hole. Yes it's bad and we can all see that any inclined bum could finger fuck the bejesus out of that thing as it traipsed down the road. Unless you're really proficient, and I mean really proficient at welding, I would leave that shit alone, you can seriously warp the hell out of that metal without much spark. Better to get a new panel, have it spot weld in and have the old one taken out. If they're going to paint it too for that price, dude your in business, Cause I reckon just $75 is what you spend on some paint that was matched to the factory color. Don't bondo that hole, bondo rely's on underlying metal for support, and as soon as the whether changes that stuff will go through thermal cycling, moisture absorption etc, and then fall out. Given the size of that hole I would say the whole panel have some permeation of rust damage on the interior of the trunk, bracing etc.