Any flat file or AS400 integration is going to require custom work so you need to expect a level of upfront and ongoing costs associated with it. You will require that piece of middleware created that sits between the Shopify API and the AS400 flat file system.
Give Virtual Logistics a shout - they would have experience with this kind of integration.
Hi, we at VL OMNI noticed this inquiry was still open and want to help out. If you are looking for an agile and scalable integration between your AS/400 ERP system and Shopify or Shopify Plus, check out our integration connector at https://vlomni.com/connector-as-400-shopify-plus-integration-application_interestas400/