Hi!
I know this is an old thread, but thought I would post incase anyone else has the same query.
I have had the same request from a client - to integrate EPOS with Squarespace so the stock inventory is updated when online sales are made. I have just contacted EPOS who have advised that this is possible. There is a one time integration charge of £999 +VAT, but they will then provide an API and their developers will guide you through how to set up the integration.