Mercedes dealers have always charged very high prices for repair work. One of the reasons they do this is because the typical dealership has a very high overhead rate. For example the dealership apparently provided you with a "free" loaner car for 3 days - these benefits are paid for by the consumer in the form of high rates. I'll grant you that $300 for a blown fuse is a bit excessive but the real question is if you suspected the fuse was blown why didn't you check it before you callled the dealer looking for help? It would have taken you less time to check the fuse than it did to make the phone call seeking assistance.

Check the simple things before you contact the stealership.