We are a small group of people working in an office not too far from the commercial area. It usually takes me no more than 5 minutes of walking every day to buy some food and such from the store. However, I've had requests most of the time to also purchase things for colleagues. I did it a couple of times but it's becoming a habit and I don't like it. For a brief period of time, I stopped going.
Today, I went out again, asked the manager what I should get and then I received a phone call from one of my colleagues asking me to buy something. After I returned with the goods, one of my colleagues said that if she knew I was going out she would have also given me things to buy. My response was that I'm not the designated courier and everyone should feel free to get their own groceries. Although I handled this situation badly,
How can I politely reject such requests in the future?