How can I create a zero-dollar invoice for a free product?

[carmelunder]

so I have a product I'm giving for free to my customers, the value is 39 usdand they get a coupon of a 100 discount. 
still, i need an invoice for the packing and sending the gift on DHL, how can i create the invoice on 0 usd? 

[YOD_Solutions]

How about adding a new item line in the invoice: Item "Discount", unit "1", price "-$39", so your total amount would be $0 (in other words, it would be a sum of product price $39, and discount -$39, total equals $0)