Rightly or wrongly, I increase my quote to Aisan sounding customers in order to have leeway to negotiate. They have strong family ties and values, and part of that is to get "the best price", so if they can beat you down, they keep face. If you have built in the margin to let them do that, you can get the price you want.
As for being watched the following works.
Go to the van, put on gloves, goggles and a respirator, which makes me sound like Darth Vader when I speak, and bring back a pump spray with somthing strong, like water. I then tell them that its probably quite safe as I spray the water around watch them leave quite fast.
Cheers
Mike