SHIPPING COST BY NUMBER OF ADDITIONAL ITEMS

[sct]

I'm running 1.7.2.4 and I am looking for a way to incrementally increase my shipping charges per item. For example, a customer in the U.S. buys one item and pays £10 shipping. If he buys 2 items I need to raise the shipping cost by £1.50 to £11.50. I can't do it by weight and I can't do it by item price, it has to be by number of items ordered. Is there a module I can buy that will do this? It really is a deal breaker for me as I sell on multiple platforms and my shipping policies need to be the same everywhere, for example eBay allows me to do this. Surely I cannot be alone in this requirement. 

I want to stay with Prestashop, but not being able to do this is costing me money so I'm thinking of moving to Opencart 3 which DOES allow me to do this

Edited December 3, 2017 by sct (see edit history)