Hi Ramune,
In fact, I made a work around to issue promo codes and print on the receipt
I created a product (Coupon Code) with 0 retail price tracked by serial number on sales transactions only
To auto issue the coupon upon purchasing an item, I linked this product to the item purchased .
When selling the item, the POS auto issue the linked product (which is the coupon code product) and asks for the serial number ( which is one of the pre-defined promo codes in the system)