You can use a macro expression. The field names originate from the DiscountCoupon class (database table COM_DiscountCoupon).

Usage in the invoice template, therefore, would be:

{% DiscountCoupon.DiscountCouponCode #% } 

for the coupon code, and in a similar way for getting other fields:

DiscountCouponDisplayName (probably the most informative), DiscountCouponValue, etc...
 

-zc-