what would you do??

[C-10volvo]

a month ago i got my manual transmission serviced by Russ Chevrolet on my 2000 cavalier. however today as i was on my trip home from campus, i head a "rock" hit the undercarriage of my car. i didn’t really think anything of it until a light white smoke trailed my car. i pulled over to inspect if anything was wrong and see a steady stream of red gear oil pouring out of my transmission. i immediately got back into the car and limped two blocks to my house.
at first i thought that "rock" may of cracked my transmission case but after further inspection, my drain plug was missing... infuriated i called the dealership and explain my position and what THEY should do about it. they offered to pay for the plug and oil to get it over to the dealership (i live in Corvallis Oregon, about 70 miles from Tigard where i got it serviced). they said they would cover the bill for anything else wrong.
however they want it checked it out at their dealership. this to me sounded sketchy. what are my options? has anyone else been in this position?