Add a standard payment on orders

I have been playing around with opencart and have learnt lots of stuff. I think it's a great program. I have a problem and if anyone has an answer I will be greatful. I am designing a gift shop. On each order e.g total order= 10 items (total cost = $100) I want to add a standard fee say $ x. This is like my service cost. I have tried using options and I thought I was successful until I noticed what I did could not help much. I had put an option to add $ x but I noticed on checkout this adds $ x for each item bought rather than the total order. How do I go around this. I think one way to approach this is add a code in the files product.tpl, product.php and product.css but I can't move past that. Thanks in advance

add a flat rate shipping extension and make it invisible to the user in the shipping selection screen.

Thanks but this cannot help. I am giving shipping as an option also so I cannot use the flat rate as well as the zone based shipping together, these will not be all selected.