This is not a FULL Expat package

They just offered you a lump sum.  A full Expat package would include base salary, hardship, house allowance, food allowance, school for children, annual home visit to US, international medical insurance, VISA assistance, Tax equalization. 

US citizen stays in China more than 183 days would have to pay local tax, the rate is quite high.  You either pay China tax then claim credit when you file US income tax, or the company will pay your China tax and you pay US income tax through tax equalization.