ShopTab has many clients working with the OpenCart platform. You have two options in terms of providing an integration for the updating of the products in your ShopTab Facebook store.
1) Create a CSV or XML file from an export of the database. This would require work to export the file and put it in the designated file formats
2) OpenCart Extension - we have a number of clients that use the extension listed on the OpenCart website. There is a one-time cost of $10 USD and details of the version supported are provided on the page. We've found their support team to be very responsive to our queries.
Once you set that file, we just need the location of the file placed in the Data Feed section of ShopTab's admin and we'll go grab the file and update the store per the scheduled upload times based upon your ShopTab Package.