Amendments to PHP web site

I need someone to change the PHP of an existing web site which calculates the cost of various products. I need to add postage costs to the shopping cart - before it directs the customer to PayPal. The postage cost is to be based on the geographical location of the customer. Here are the 5 options and amounts I need to add to the final cost: Australia $10.30 New Zealand $14.80 Asia Pacific $18.40 USA, Canada, Middle East $$21.85 Rest of the world I will give you the address of the site if I think you can help. I need this done by next week - Monday if possible Thanks Hamsta