first-time-customer-discounts.php

<?php

// If next year is not yet here...
if ( (int) date( 'Y' ) < 2022 ) {

    // ... then give a 50% discount on the initial item price...
    add_filter( 'wpinv_get_initial_item_price', function( $price ) {
        global $wpdb;

        // ... unless he/she is not a first time customer.
        if ( ! get_current_user_id() ) {
            return $price;
        }
    
        $paid_invoices = $wpdb->get_var(
            $wpdb->prepare(
                "SELECT COUNT(*) FROM $wpdb->posts WHERE post_status = 'publish' AND post_type = 'wpi_invoice' AND post_author = %d",
                get_current_user_id()
            )
        );

        return $paid_invoices > 0 ? $price : $price * 0.5; // Give 50% discount if a first time customer.

    }, 10, 2 );

}