Dynamic Orderlist with PayPal IPN feature

We need an expert to create a dynamic orderlist for our existing website. It must be a PHP script with AJAX technology. The orderlist is equal to a shopping cart where the user can add and delete products. All products have the same price but different item codes and the cart must also put in consideration a dynamic discount structure (i.e. quantities 1-5 are 'X' amount each, quantities 6-10 have 15% discount and are 'Y' amount each and so on.) which must be adjusted without page refresh. The customer must be able to delete a product and the product image in the cart must be link back to the original product page. The products are of a digital nature and NO physical delivery needs to be put in consideration. It must have also the option to apply a dynamic 'promotion code' which allows to deduct 'X' amount of products free. On 'Checkout' a summery of the user's order must be presented and after approval by the customer the amount must be sent to PayPal for payment and after payment to return to the website's 'thank you' page. The customer order must be inserted into the website's database after completion of payment from PayPal. The above application must handle also the communication with the client such as issuing an invoice of his/her order and instruction where the digital products can be downloaded from our website. The order list(cart) should have its own external CSS file. You should have experience working with PayPal's sandbox and IPN features. The application must be working well on OUR server/website before any ESCROW deposited money will be released. Server is: LAMP Database is MySQL (already existing but new tables must be added as required with the above application) We provide full FTP and database access but NO control panel access. Full co-operation in any issues are provided by website owners. Any question regarding the above specification will be handled to our best ability. Time frame: A.S.A.P. thanks for your interest