Allow payment from WC generated invoice without login

functions.php

/* Allow payment from WC generated invoice without login
* @see https://wordpress.org/support/topic/order-pay-without-login/
*/

function allow_payment_without_login( $allcaps, $caps, $args ) {
    // Check we are looking at the WooCommerce Pay For Order Page
    if ( !isset( $caps[0] ) || $caps[0] != 'pay_for_order' )
        return $allcaps;
    // Check that a Key is provided
    if ( !isset( $_GET['key'] ) )
        return $allcaps;

    // Find the Related Order
    $order = wc_get_order( $args[2] );
    if( !$order )
        return $allcaps; # Invalid Order

    // Get the Order Key from the WooCommerce Order
    $order_key = $order->get_order_key();
    // Get the Order Key from the URL Query String
    $order_key_check = $_GET['key'];

    // Set the Permission to TRUE if the Order Keys Match
    $allcaps['pay_for_order'] = ( $order_key == $order_key_check );

    return $allcaps;
}
add_filter( 'user_has_cap', 'allow_payment_without_login', 10, 3 );