Example CSS to only show the invoice for print media on the PMPro Confirmation page.

pmpro-confirmation-adjust-invoice-print.css

@media print {
    /** Hide stuff when printing **/
    .pmpro-confirmation .site-header, 
    .pmpro-confirmation .pmpro_actions_nav, 
    .pmpro-confirmation nav,
    .pmpro-confirmation header,
    .pmpro-confirmation footer {
            display:none;
    }

    /** Only show this to print, tweak to only print certain ID's or classes on the page **/
    .pmpro_confirmation_wrap  {
        display: block;
    }

    /** Hide confirmation message paragraphs, tweak to suit your site **/
    .pmpro_confirmation_wrap > p {
        display: none;
    }
}