When you go to a buffet in a restaurant, think you are offering to the guru deity, thinking the food is not for you. It helps you not to feel you possess it, that this is mine. This helps you to practice the bodhisattva vows. This is also an antidote to attachment, and when you practice like this there is peace in your heart, and happiness. Otherwise, if there is attachment performed with ignorance, then every single thing becomes negative karma, whatever you do.
Similarly, when you do shopping, watch and prepare your motivation. When you are in the shop, either do it to fulfill the wishes of the guru or to benefit and serve other sentient beings. The ultimate purpose is to fulfill the guru's advice. So, you buy these things to survive so as to fulfill the wishes of the guru or benefit sentient beings. This way it becomes an antidote to attachment, becomes a virtuous activity, Dharma, and collects extensive merit.