Change Postage Message

Support,
could you please look into a way to allow us to have our own message show if a valid postage is not selected. We need to be able to limit overseas Order size to £250 but be able to show a message to show that we accept overseas orders over £250 but would need to quote them for the postage rate dependant on weight/country. At the moment Roman cart message comes up which does not provide the message we want and we can see no way of changing it. Is it possible please to add your Roman Cart message that is displayed to the change any text area so we could then change the message given?
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Support
the Roman Cart message that we want to be able to change reads,

There is a problem with the selected shipping method or the merchant may not ship to your area. Please choose a different Shipping Method or review your location if applicable.

we cannot see that on the change any text screen
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Thank you that helps us a lot
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