Hello Bernhard,
It should not be terribly difficult to integrate these apps with each other because they are built on the shared Common Data Service. However, there would be some development work involved but not a lot. It is worth looking into. I believe you should be able to find some documentation for the same.
Additionally, I would also suggest that you check the Microsoft AppSource for some e-Commerce solutions that integrate directly with your Dynamics 365 Apps. The best part about third-party programs on AppSource is that they undergo an in-depth approval process with Microsoft before they can be listed, so you always know that you are working with software that you can trust.
Regards,
Chris