I must be missing something here. Four to eight hours to change out an automatic in an old mercedes 240d? Talk to a reputable transmission only shop about a time and charge to pull and install one. Ask around. Of course he wants to get his normal source to rebuild it as well as he gets a large cut of your quoted cost. He might even get a smaller cut if he orders it as a functioning auto repair business from sun over your price. Thats why one poster stated about 1100.00 from sun I think. Email that poster and ask. You might try that angle to work the price down. Tell them what you want done exactly. Get a price. I know my own time and am not a mechanic. Forget 680.00 for a change or at least discuss it with him prior to the job. At those prices he will gaurantee his future labour of course as you have prepaid for it anyways in my opinion. Perhaps for several changes even. If the old transmission was changing well once the new one is in the vacuum circuits should not have to be reworked etc. Use a quality known box like the sun unit of course. I suspect a competent transmission guy at 30 minutes out and about 30 minutes back in. Check. It probably might consume about two hours for me to do this on one of my own cars as I do not do autoimatic changes very often. Plus am older and slower now. Try not to be a victim. Basically you owe this guy nothing as I see no attempt at fairness from him up to this point. If you still feel obligated negotiate with knowledge including his cost from sun versus yours and a fair time charge for the change. That way if a warranty change is required sun might go good for it as well. A few phone calls should get things sorted out. Do not attempt to prepay labour you may not need. Keep some of the insurance money for the bull you are subject to along the path of life. Think how lucky your mechanic is that he is not dealing with me. I was just getting warmed up.