"Saving" Orders for clients to return to a pay

I am build a normal ecommerce site of products, shopping cart, etc. But when a client gets to the "Invoice me" or "Credit card" payment screen, I would like them to have the option to "Save" the order. Then with the link they receive in their email, they can return to the order status screen and click a link to pay. Another way this could work is they receive a link back to the payment screen to choose how they will pay, then pay and complete the order.

Also I want to use this for my CSRs to create orders for the clients while on the phone, then when the clients get the link in their email, they can click into the order and pay.

Are there any modifications or add-ons I can use to make this happen?

THANKS.