Change PayPal Description?

I want to add a function that changes the Description (field label) sent in the PayPal post. I have a function that changes the price value depending on the type of post the user is paying for. So I would like the type of post to be sent in the description. Would I use:

<?php
add_filter("gform_product", "change_product", 10, 2);
function change_product($label, $form_id){
//find users post id
//use id to find post type
$post_type = //post_type;
    return $post_type;
}
?>

I am currently getting a distance for the user that is posting a housing listing and depending on how far away they are from a certain point the price changes. So in the paypal details I would like to include the distance. I am using:

add_filter("gform_field_value_price", "populate_price");
function populate_price($value){

	$id = get_current_user_id();
	$args=array(
 	 'author' => $id,
 	 'post_type' => 'listing',
 	 'post_status' => 'pending',
 	 'numberposts' => 1
);
$my_posts = get_posts($args);
if( $my_posts ) {

$post_id =$my_posts[0]->ID;
}

	$distance = get_post_meta($post_id, 'distance', $single);

	if($distance[0]>10)
	{
		$price = "$50.00";
	}
	else
	{
		$price = "$35.00";
	}	

   return $price;
}