Wrong currency in checkout

Hi! When a customer places an order with an adress outside Sweden they get to pay extra for the shipping. But I have found a problem. When someone in the US places an order they should only be able to check out with USD. But one customer abroad used the Swedish Krona and in the checkout they paid 54SEK instead of 54UDS (this is a big difference)… How can I ensure that a person with at shipping adress in the US can only place an order with USD and so forth?

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