Customize Opencart Invoice Feature

Hi, I need someone expert to help me customize this. 1. Allow order edit, such as add/edit/remove product,options,price,shipping,voucher,coupon etc. Or if your modification can work with this extension I'm fine too as I've purchased it: [login to view URL] 2. Once order is edited, the invoice number and existing total has to be remained, but will add new rows at the bottom of the invoice (as attached image). 3. Any stock control deducted or added to the order should counted in the inventory system as well. Such as T-shirt A is removed from the order, so will return QTY 1 back to the product T-shirt A. T-shirt B is added to the order, so QTY 1 will be deducted from product T-shirt B. 4. Order details can changed to the new edited one, however the invoice must follow the format as attached. Stock Exchange (explanation) - Customer decided to switch the product, he no longer want LS4012, so RM69 is deducted out. - He wishes to change to another item which cost RM59. Below this is the new calculation of the new total. Kindly bid if and only if you are able to do this without causing any error/bug which will affect the functionality of the site. Quote me your best price as well. Thank you!