Calculate Shipping [enter zip here] [enter country here] [select shipping option here]
Is this on the product or cart page or both? Do we assume we can use these details to pre-populate the actual shipping form later?
Though I agree that your simple example can work in certain limited circumstances and is something I would love to see on most sites I visit, shipping is one of those things that gets very complicated very quickly.
Edit:
Come to think of it. A simple and persistent form displayed on each page that I could fill in at the start of a shopping session that allowed the shipping cost to be displayed alongside the current cart total at all times would be pretty useful. I don't think I've actually seen an example of that.
That's all you should need to do. Or have a flat rate for e.g. continental US.