Custom wp-e commerce checkout

We need to build a custom wp-ecommerce checkout system. Here is how it needs to work. 1. add to cart, item added to shopping cart 2. check out -> takes you to the check out page. - payment option : paypal & credit card (eway). Credit card number has to be validated. Payment gateway has to be set up properly. 3. Once transaction has gone through. The system puts transaction on pending. An auto responder email is sent out to the customer "Thank you for your purchase, will get back regarding availability". Another automatic email is sent to the store for the staff to check on availability with two options : approve transaction or cancel transaction. transaction will not be processed until it has been approved. This needs to happen in case an item is no longer as it ay have been sold in store. 4. If the approve button is selected, payment is released from pending and automatically processed. Another auto responder email is sent to the customer informing the success of the transaction. If it is rejected, transaction is cancelled, customer will receive and email "we apologize for the inconvenience... "