Only ship to the USA when dollars are selected

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Hi there,

I am trying to restrict the shipping options depending on the currency selected. For example I will add a button on my site that says IF YOU ARE FROM THE USA CLICK HERE this will then switch the currency to dollars (set at my own rate). now when people progress through the cart I want them to only be able checkout if they have a US address, if their address is from anywhere else I would like to display a link to the international site.

Any help would be gratefully received.

Thanks
Matt