I would definitely read up on the Shopify and Magento API's. How are they implemented? Is there a script you can call via AJAX, or via a cronjob/cURL?
What about the Database back-end for the RMS ... can you have the site access that? There are often several ways to skin such a cat. The trick it, knowing ahead of time what problems might be encountered on the way to Nirvana....
And Kudos for trying Opencart. I've no experience with Shopify, but Magento is a 400 pound rhino ... big, slow, and sprays s**t everywhere when you wake it up ... 😛