Migrating from Opencart to Drupal Commerce

I don't know Opencart, but for complex migrations you might be better of with the Migrate module. That module is specifically designed for migrations and has better support for relations between entities. It does have a steeper learning curve, though. While there are UI modules available, you probably need to be a coder to get such a migration done.

If you only want to migrate a series of products that are simple structured, you could export these from Opencart to CSV (assuming Opencart supports that) and import them with this module and Commerce Feeds.

See also the project page of Commerce Migrate that explains when you could use Feeds for a migration and when to use Migrate.