Perfex CRM Customization

Hello I have purchased the Perfex CRM (Codeigniter) from Codecanyon, I require a small amount of customization work to make the application perfect for our business needs. We are looking to add a simple stock control. We want utilize what is already build in so job will require to add few columns to database and a bit of coding. Please find Perfex CRM demo on the internet to follow our requirements. 1. Under Sales -> Items modify ‘Add Items’ form to include tick box ‘Trackable Inventory’. When ticked it allow user to add additional data (quantity, unit cost, location, serial number). 2. On main screen Sales->Items add column ‘Qty’ that will show value from items quantity field. 3. Small customization in Invoice screen (Sales->Invoice). Basically when adding products/items from build in dropdown to show quantity next to the item (screenshot attached). When such a ‘trackable item’ is added into invoice to and invoice is saved than to substracts items qty added onto invoice from items list. Eg. I have 10 LCD TVs in my Items list. I create new Invoice and from dropdown select LCD TV item and change qty on invoice to 3. I save invoice. In that moment my LCD TV stock on Items list should be updated to 7. When I cancel or delete such an invoice all items and their quantities should be updated on Items list. Eg. Cancelled invoice with 3 LCD TVs on it, so my qty of LCD TV item on Items list is changed back to 10. Project described to my best. Please bid real fixed prices. Thanks