prestashop modifications

I am using a copy of the default prestashop 1.6.x template. I need the following modications on the cart (ajax one page checkout cart): - Add the following - Line for Packaging weight (as shown in the screenshot) determined by the following formula: If the weight of all products is in the ranges below then add package weight line = the number in grams. if total of products weight 0 - 50 then add extra weight of 10 if total weight is between 51-100 then add 30 if total weight is between 100 and over add 50 - Add a colum in the cart that shows the price per unit (or unity/unity ratio) -Ajax function if the products in the cart are changed or quanities are changed, then update the total weight/packaging weight without reloading the page, this cost should also be reflected in the blockcart -Also on the cart page, have the payment options displayed all the time regardless of the location, I use paypal/bank and check. I do not want the user to have to enter a address before they are shown. --------------------- All code must be in overrides folder or in template folder and should not effect the functionality of the existing layout (i.e. please dont break anything and I dont want to loose when I update my prestashop). You should also let me know where it has been added (i.e. file name and lines) in case I must rollback or upgrade. ----------------------