Just to follow up the story...
I went to an independent shop to check if they have drain plugs in stock before I try the dealers. No, they don't. But I was told to try the lube shop next by. Surprisingly they did have the right sized drain plugs that fit my C280. They didn't expect to see the plug that was on so tight, but they managed to untight it at the second try. They unplugged the old one and put the new one in. During the process about 1/2 quart of oil was drained, but there was still enough oil in the engine to drive home. I was charged $10 for part and labor, which I think is very reasonable. The new plug seems to be of good quality. Of course you can buy one from dealers and ask the local lube shop to put it on. Anyway, it was a great experience and then I enjoyed my oil change once again.
Just a reminder to myself: I will never let others change oil for me again!
Harry (94 C280)