I don't usually have any problems with getting the customer to not be upset about custom charges. I do try and give them an informed choice, and explain the details as clear as possible. I explain to them that I am not personally taking any money from them, that my HSP will be processing the charges. (which helps with thinking "Oh he is trying to make an extra buck on this or that") I also keep actual quotes from Electricians, the cable company and phone company as to what they charge for similar work, so they can see its considerably cheaper, and make a decision if they want me to do it or someone else in another trade.
I always offer my Cell phone number for future questions and other non-billing type of issues. Its really about the presentation. I had a guy once with 3 HR20's, and 4 D10's have no problem paying me $55 for a fish for each line, and I was the 3rd tech to quote him the same price, because of the approach I used to explain to him why it wasnt covered in "Basic" install. The fishing didn't take much time at all, and after everything was said and done, agreed to, when I called in charges, I didn't charge the $110ea for the DVR lines, since he didnt have any firebreaks where they were going to be placed. After everything was said and done I saved him $165 over the $550 originally agreed to. While closing out with him our conversation lead to him to saying that eventually he wanted to put a LCD in his detached Workshop/Garage, and found that the electrician left an open/unused 3/4" conduit from the meterbox to the garage, so I offered to fish him a future line for free before I left, took all of 10 minutes to do. Long story short, I ended up getting a $100 tip and some coupons for free oil changes at his business, and he personally met with the president of my HSP to commend me, which I ended up getting further bonuses for. All this and I did nothing more or less than I do for anyone else.
Boils down to being confident, professional, educated and courteous to every customer whether they live in a $40,000 home or a $5,000,000 home.