Allow WooCommerce existing customers to checkout without being logged in [closed]

You would allow the user to fill the email, use it to find the user id at the database and then use the id for the order.

One possible solution is to replace function wc_create_new_customer( $email, $username="", $password = '', $args = array() ) with a custom function.

This is where the function is called, all your custom function needs to do is find the correct user_id.

    protected function process_customer( $data ) {
        $customer_id = apply_filters( 'woocommerce_checkout_customer_id', get_current_user_id() );

        if ( ! is_user_logged_in() && ( $this->is_registration_required() || ! empty( $data['createaccount'] ) ) ) {
            $username    = ! empty( $data['account_username'] ) ? $data['account_username'] : '';
            $password    = ! empty( $data['account_password'] ) ? $data['account_password'] : '';
            $customer_id = wc_create_new_customer(
                $data['billing_email'],
                $username,
                $password,
                array(
                    'first_name' => ! empty( $data['billing_first_name'] ) ? $data['billing_first_name'] : '',
                    'last_name'  => ! empty( $data['billing_last_name'] ) ? $data['billing_last_name'] : '',
                )
            );

            if ( is_wp_error( $customer_id ) ) {
                throw new Exception( $customer_id->get_error_message() );
            }

            wc_set_customer_auth_cookie( $customer_id );

            // As we are now logged in, checkout will need to refresh to show logged in data.
            WC()->session->set( 'reload_checkout', true );

            // Also, recalculate cart totals to reveal any role-based discounts that were unavailable before registering.
            WC()->cart->calculate_totals();
        }

Hope this helps to point you in the right direction.

Cheers,
Gabriel