Again it’s a natural thing, we cannot refuse something for nothing, even though we know subconsciously there is no such thing!
Cash or % offers are long established, without doubt they attract attention. Used occasionally such as to boost trade in quiet periods can be effective, long term use however is destructive. It can be impossible to get back to normal pricing.
If we get an email offering us two main meals for the price of one, it’s likely to tempt us out for a treat. The secret of success is to offer something that the customers sees as a benefit whilst not losing money on the offer. Restaurants aim to claw back profit on starters, deserts and drinks.
Offering a service as a way to open doors can be a good tool to invade your competitors territory. Free safety checks are a good example. Like many marketing tools, returns my take time to deliver.