getpaid-display-discount-code.php

<?php

// Add discount column.
		add_filter( 'manage_wpi_invoice_posts_columns', function( $columns ) {
			$columns['discount_code'] = __( 'Discount Code', 'invoicing' );
			return $columns;
		}, 999 );

		// Display discount column.
		add_action( 'manage_wpi_invoice_posts_custom_column', function( $column_name, $post_id ) {
			if ( 'discount_code' === $column_name ) {
				$invoice = new WPInv_Invoice( $post_id );
				$discount = $invoice->get_discount_code();
				if ( ! empty( $discount ) ) {
					echo wp_kses_post( $discount );
				} else {
					echo '&mdash;';
				}
			}
		}, 10, 2 );