Likely the weather stripping is very custom to your Model S and only available from Tesla. Have you checked the price of just the part from Tesla? I've generally found the parts prices like this to be reasonable. My guess is they are charging a one-hour minimum labor to replace it. If the inside door panel and glass need to be removed, it could take more than an hour. I'd go to the service manual to get the part numbers and see what effort is needed to do the work - might be a lot more than you expect.