Since price and tax rate are specified for the product set item, no individual tax rate is calculated for each product set item (for example, weighted by sub-products). The price of sub-products is shown on documents as 0.00 € and is hidden on Shopware standard documents together with the tax rate if you have set in the plugin configuration that sub-products should be shown on documents.
Accordingly, a suitable tax rate has to be selected for the product set item. In most cases, this is the highest tax rate of the sub-products. In certain cases, however, a lower tax rate may be possible. For more information, please contact your tax advisor.