Prestashop custom code.

for specific shipper, I need to add some custom condition for shipping. for example, I put rate table for specific shipper(ID: 51) like this (unit is lbs) 0 - 1 : $1 1 - 2: $2 2 - 3: $2.5 3 - 4: $3.5 4 - 5: $4 ..... but int certain condition, I want to give some discount ** Condition is like this If Shipping Amount is bigger than $100, 1) under 2lbs: free shipping 2) over 2lbs: original rate - $2 (changeable) case 1) When user put items into cart, total $90 and 1.5lbs, shpping charge will be $2 but user put items into cart, total $100 and 1.5lbs, shipping will be free case 2) When user put items into cart total $90 and 4.5lbs, shipping charge will be $4 but user put items into cart total $100 and 4.5lbs, then shipping chaarge will be $4- $2 = $2 ------- I don't need installable module, just updating code is OK.