example:
as of v2.11.8.6 you can add this to your themes functions.php if you would like to display “free” for example if price would otherwise be displayed as 0.00 (note to self: perhaps make that an option at some point)

add_filter('wppizza_filter_output_format_price','my_function');
function my_function($priceformatted){
/*get rid of any formatting first and cast to  integer*/
$price=(int)str_replace(array(',','.'),'',$priceformatted);
/*if zero, display some text instead*/
if($price==0){
$priceformatted=__('free','wppizza');/*set this to whatever text you need to be displayed*/
}
return $priceformatted;	
}

PS: not 100% tested, but should work. if you have any issues, let me know.