I think I may have found a reasonable solution:

Under the postage calculator there is an additional service called "Additional post to locations - buyers contact for costs" with a selection of countries. I am presuming that if I remove US from the Calculated postage option and add to that "additional post to locations" option, my items will still appear to US buyers in search but won't give them a postage cost calculation and they will be forced to request a finalised invoice before they can pay. Just wondering how I can check that.. Has anyone used that option?