My boss keeps asking me to buy sweets for her?

I've started yet another temping job and I always bring in a big bag of chewy sweets to break the ice any where I work. My boss at the moment in this job has taken a shine to these sweets and asked me to buy her 5 packs (she gave me the money of course). I did so and although they were heavy to bring in, I did it. She doesn't live anywhere near the type of shop I buy them from and now she wants me to get her 10 packs! I think this is a bit of a cheek. How can I avoid this?