Mechanism: add just HK$1 when you spend HK$90 on cat food in order to get a gift with purchase. The gift is a Cat Brush & Cat Scissors packed together in blister pack. Both are high in utility value and perceived value for pet owners.
This promotion is technically called a PWP (Purchase with Purchase) campaign. However, given that you only need to add around US$ 0.12 to the purchase it really is just a token contribution towards the cost of these gifts.