Currently you must create a duplicate product as your bonus item. Issue being that if your are doing a Buy 1 Get 1 Free deal, your inventory of that particular item will be thrown out because the bonus items actual inventory is that of the target product. When you Sell 1 target product and include a bonus item, your inventory will be -2, but the shopify inventory will be -1 of target product and - 1 of the duplicate.
Link the target and duplicate counts. Right now it would be a manual operation. For each deal taken up you will