You have probably already seen it but Realsable IMan is by far the best option (https://realisable.co.uk).  All the hard work is already done in the connectors they have for Sage 300 and Shopify.  It's a superb product.  

Yes you create your own integrations but it's so easy to do.  It's not coding development.  For example, I have created OE order integrations (Shopify, etc to Sage 300) in about an hour.  There are advantages to this as well given the inherent flexibility.  There always seems to be something "special" about every integration.

Please take a look at our solution workflow:

https://mailchi.mp/selessoftwaregroup/shopify-integration