How do I add "ADD TO ORDER" to an invoice?

[Hankatclover]

I would like to send an invoice to customers for items they have ordered, but enable them to add MORE to their order. Currently, when I send an invoice, customers can just pay for the invoice and we're done. I'd like for them to "Add to cart" or "Add to Order" or enable the customer to  shop and add to their order. Help!