contact form 7 use javascript to sum and retrieve the data with shortcode

Hi,
first of all , I would like to thank the creator of this great plugin
Currently , i made a form and i need to make a sum in this form
To make it possible i create a file and used javascript
Unluckly i can’t retrieve this sum in the mail
I wanted to create a shortcode to make it possible but my knowledge in php and javascript is a bit low

What i would like is a form that when i ” check” two boxs i can have the sum of these 2 checks as a value and retrieve this value in the mail like all the other tags made thank to contact form7

At the end , when the user click on submit
i would like to retrieve in a mail
His name, adress, tel, sum of these 2 checkboxs checked,

I don’t know what i explained is conmprhensible
I hope So

Thank you for the help