<?php

add_filter( 'woocommerce_gzdp_shortcode_invoice_data', 'my_child_invoice_due_date', 10, 3 );

// Add [invoice_data meta="due_date"] as shortcode to your invoice settings
function my_child_invoice_due_date( $return, $atts, $invoice ) {
    if ( 'due_date' === $atts['meta'] ) {
        // Calculate date
        $date = new DateTime();
        $date->setTimestamp( strtotime( $invoice->post->post_date ) );
        $date->modify( '+2 weeks' );

        $return = 'Rechnung ist zu zahlen bis zum ' . $date->format( get_option( 'woocommerce_gzdp_invoice_date_format' ) );
    }

    return $return;
}