How to set shipping cost for additional items in prestashop using USPS

[alirm]

Hi guys,

I have tried to figure out how to set additional cost for second item in the shopping cart but I couldn't find any way in back office!!
Well, here is my case:
For example,
I have set shipping module for USPS for per item 1 LBS = 5.50 based on USPS price rates. But if some one purchase 2 items in the same time then shipping cost will be 11 $.
The thing is I don't want that my customers pay 5.5 $ for second item and I would like to set only 1 $ per additional item in shopping cart.
I think the prestashop can not support a feature like that!!!
The thing is we have such feature to set our own additional cost for shipping in Amazon store for per additional item..

I will appreciate it if some one help me and answer my question to set desire amounts for additional item in shopping cart.

Thank you