Commerce Invoice Entity Print Styling
What I found out is:
Every available printing engine has it's own requirements for the HTML-structure of the generatred invoice.
TCPDF needs a header and footer-tag inside the document. If you dont provide this elements, TCPDF will render them empty, but will in fact attach an hr-tag after the empty header and before the empty footer.
I switched to DOMpdf as printing engine, which also offers the possibility to define a page header and footer, but ignores them when empty and doesn't render any additional HTLM by default.
pro: DOMpdf supports more CSS rules and makes it easier to style the invoice with CSS
con: DOMpdf takes some seconds to render/print the document from the HTML, while TCPDF is very fast.