We use OpenCart too. OpenCart was once supported by Linnworks through their Generic Website Integration, which has now been dropped I think. We already have the Generic Integration set up so it's still vaguely working for us.
For the bits that don't work we use an import/export module for OpenCart which drops CSVs into an FTP, and then we have scheduled Imports and Exports in Linnworks using the CSVs on the FTP. This works reasonably well as a workaround.
The module we us in OpenCart is "Universal Import/Export Pro" made by Geekodev.
We use open cart as We have a very terrible experience with Woo commerce. Woo commerce is just a waste of time. If you use open cart it is so easy and very very much professional. Opencart is very clean and very friendly. The customers are extremely happy with our new website. A lot of features are free. You dont have to pay for mobile menu as it comes free, Many other important features are free.
The problem is Linnworks dont have direct integration, E business Guru did the integration for opencart and you have to purchase it 30 Pounds Per Month Which is very expensive in addition they are very unprofessional. We spoke to them 7 days before and our website is still not integrated as they dont know about " Open Cart End Point" option without which you cannot integrate channel.
Do you guys have the same problem as well..How to get Open Cart End Point value.?