Itemizing you invoice will only lead to unhappy customers when they take your invoice and start pricing each and every item out on the Internet (and of course, they'll forget how much it costs to ship heavy stuff like cable) and "conclude" you "ripped them off" royally.
You're not allowed to make any profit on materials, despite your having to make a trip to the supply house (paying $2.75 a gallon for gas, plus wear and tear on the vehicle) or putting your money up to keep a stock of supplies.
I think you're doing the right thing now, and as LK notes, if it works for you, stick with it. You might even be better off by not even separating labor and materials to avoid the "curiosity" factor, or the "$20 for parts $200 for labor" thing.
Just my 2 cents worth (not itemized).