Add a charge to the bill from predefined values

[cyberdoc]

hi guys, i basically wanna add a fee of installation of my products for the customers. So for this ive made a module to add a custom field in the products tab in the backend (where admin can set the installation charge), save this value to the db etc. 

This all part is done and working nicely.

now i want to just add the installation charge to the bill (the value of installation charge saved in the database). And also show this in the step one of the checkout (where products are listed).

How do i proceed? 

any help would be great.. which hooks should i use?