Change WooCommerce $0 zero price output from Free! to something else.

functions.php

<?php
// Do not copy the opening <?php tag above

// Customize the Free! price output
add_filter( 'woocommerce_variable_free_price_html','wsm_custom_free_price' );
add_filter( 'woocommerce_free_price_html', 'wsm_custom_free_price' );
add_filter( 'woocommerce_variation_free_price_html', 'wsm_custom_free_price' );
function wsm_custom_free_price( $price ) {
	// Edit content between single quotes below to desired output
	return 'Prepaid Inventory';
}