Develop a magento module.
1, customer add some products to shopping cart, place order, but not pay it immediately.
2, Several days later, the customer come back to our store, login, he could view his orders in the account panel. There's one action "Pay" after the pending order he placed last time.
3, customer click "Pay" action, redirect to payment method select page, he choose one payment method, click "Pay Now", redirect to third party payment gateway (ie. paypal) page.
4, customer pay the order, the order status update to "process".
Here's it. Please leave your quotation. If you have any questions or ideas, please contact with me, thanks!