Total price = fixed amount + to be determined

Hi,

I have a question w.r.t the price and payment. In my case, the shipping cost is a variable, and can only be determined at a later on stage during the order process.

Therefore, I would like my total price = fixed amount + to be determined (shipping cost), and as a result,

1) the user will pay for the fixed amount during check out;
2) after receiving the fixed amount, the order will be further processed, and the shipping cost will be calculated; and,
3) the user will be asked to pay for the shipping cost, and then the products will be shipped.

I am not sure how to proceed, and any suggestions are very much appreciated.

Thanks.