Buy two, get the cheapest one free

The cheapest qualifying item is the free one

This is the rule that decides what the offer costs you, and most merchants do not think about it until the first month's numbers arrive. A cart with a $90 jacket and a $30 shirt, both qualifying, gives away the shirt. Never the jacket.

It matters because buy one get one on a mixed catalogue is otherwise unpredictable. Without that rule the same offer costs $30 on one order and $90 on the next, and your margin depends on what shoppers happened to put in the basket.

Matching what earns the reward to what it gives

The buy side and the get side are set separately. Each can be a specific product, a whole product type, or everything from one vendor. Buy two of anything in Socks and get one free is one product type on both sides. Buy a jacket and get a free beanie is a product on one side and a product type on the other.

There is a third setting worth knowing about, because it prevents the most expensive mistake in this offer type. You can require the reward to be the same product type or vendor as the item that earned it. Without it, a shopper who qualifies by buying socks can take the free item from anywhere in the qualifying set, which is rarely what you meant.

Deciding how far it repeats

Left alone, the offer repeats as far as the cart allows. Six qualifying items become three free ones. That is correct for a genuine multibuy and alarming for anything else.

Cap it and one cart earns the offer a fixed number of times however much the shopper buys. A cap of one turns buy two get one free into a single reward per order, which is usually what a merchant means when they describe the offer out loud.

No code, and nothing for the shopper to do

The discount applies the moment the cart qualifies. There is no code to hand out, none to leak onto a coupon site, and none still being honoured three weeks after the promotion ended.

It also means the shopper stays in control of what they get. The offer discounts something already in their basket rather than adding a product they did not choose, which is the difference between a reward and a surprise.

Where it beats a spend threshold

A gift behind a spend threshold pushes basket value. Buy X get Y pushes unit count, and it does it inside one category. If you sell things people buy in multiples, socks, underwear, candles, consumables, this is the offer that fits how the product is actually bought.

If you would rather reward total spend than quantity, and give something the shopper did not have to add themselves, tiered free gifts is the better shape.