Always leave the customer something to have a dig at eg a little wheel scuff .
When they ask what is going to be done about the wheels because of the scuff , tell them its priced into the deal , BUT I can get it done for you at £100 , amazed how many forget the scuff and its never mentioned again , all because you have put the ball into their court and now they have to decide .
BUT some of the others will have it done and with these people it hasn't cost you the £100 ,
Win WIN .