Shopify > Amazon MCF > Amazon FBA

Hi All I am having issues with trying to upload Amazon MCF app to shopify (we are based outside US) and deemed incompatible, yet all our stock are in US fulfillment centres so as they are oversize we are only interested in selling within the US market for now. anybody know how to get around this issue I have also sent a note to shopify support and they suggest amazon by codisto which is a costly 3rd party app!!