About the career, need advice.

I'm 29 year olds, just graduted last year, got BBA. Two months as a Sales executive at a Thai medium size shipping line, five months as a Customer service at a Thai freight forwarding company, now I'm just start working at BPO MNC freight forwarder for two weeks as an operation officer with 18K THB basic salary and full contract but still in 119 days probationary period.

Let's get straight to the point, today I receieved a call from one of the biggest shipping line in the world, according to the conversation and the email which sent after, they offered me a position of freight assistance, 6 months contract with 20K THB Basic salary.This is an all information I have got for now.

So please kindly answer my questions below ;
1.What is a freight assistance's job description? 
2.Should I negotiate regarding a basic salary and if yes, how much?
3.Is it possible to negotiate regarding the contract type or a period of contract?
3.What should I tell my current company if the negotiation is succesfull?

They need my final decision by next monday though.

Thank you in advance for every advices. 

Ps. I have no thai keybord on my laptop now, hence sorry for my bad english.