Aha! a clue ... you mean that one ...

You could customize the products_listing module by changing the code that reads:
PHP Code:
        case 'PRODUCT_LIST_QUANTITY':
        $lc_align = 'right';
        $lc_text = $listing->fields['products_quantity'];
        break; 
to read:
PHP Code:
        case 'PRODUCT_LIST_QUANTITY':
        $lc_align = 'right';
        if ($listing->fields['products_type'] == 4) {
        $lc_text = '';
        } else {
        $lc_text = $listing->fields['products_quantity'];
        }
        break; 
NOTE: this assumes that document_product is still 4 ...

NOTE: Be sure to use your templates and overrides for this ...