Let’s say your package (or listing) costs 100 EUR. If your VAT is 19%, then invoice item unit price is 84.03 EUR and tax is 15.97 EUR (19% of 84.03). Together it is 100 EUR.
Default VAT rate can be changed in the WP customizer. If you need to set custom tax for specific order/payment, you can achieve it using
inventor_invoices_tax_rate WP filter.
Variable symbol is an identifier widely used for domestic payments. Is usually used to indicate invoice numbers, but it can also be an order reference or user id.
Lines in the invoice billing details can be anything you need. Special payment details, identifiers, address or whatever.