hi, some of my suggestion:
谁计算出海关钱?----谁计算出海关成本?
多少钱一个集装箱的成本?---一个集装箱的成本多少?
谁交集装箱钱?----谁支付集装箱成本?
你交集装箱钱和海关钱吗?---你支付集装箱和海关的成本吗?
(“支付”is more oficial than “交”, used more often in the transaction, 支付 means PAY, and 交 means HAND OVER)
1_What is the lowest amount that we can order?--最小的起订量是多少?
(I guess you mean MOQ?)
2_How many percent can you give me discount?---你能给我多少折扣?
hope I can help you.