German law requires that a tax depending on the tax of the product has to be added to the shipping costs.
So when displaying the checkout the tax given there is "wrong" at least for germany, because the tax of the shipping costs is missing. (see: card step 05 missing tax for delivery.png)
During testing i found a not so clear point in the setup (see: card step 04 backend settings.png)
Thats leads to an unwanted behavior on the frontend (see: card step 04.png) where only the first unit bought from a seller should be billed with shipping costs
For the whole process please see all attachments.