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-