Silence is gold.
You quoted a client! He or she is staring at you; you feel the need to say something more, something to justify your price. DON'T! Say nothing! Silence is gold! Literally! Let your client think about it!
If they want to bargain, let them take the first step. Hear them out. Then talk! Explain to them the added value you are offering. This is nice and simple, and best of all, it is tested! Try this out, if you're not already doing it! What do you think?