What are the best way of payment for your client who buys your consumer electronic products in large quantity, apart from "TT, deposit in advance+balance paid before delivery?"

Agreed with aseky for the irrevocable L/C at sight. But if you cannot do L/C and your company have only T/T then it will be a problem. Most of the time big company does not do T/T due to their company's policy of risk control. I think the most that they can accept is probably 30% upfront and 70% after delivery, and you are talking about large amount here so I guess if you don't know your buyer very well then it will be quite risky.