My boss constantly making me do his special tasks

So I got there as an administrative assistant, but I'm literally dealing with the plumber at the woman's house too. The other day, she sent me to the stationery store for her child's school project. It's really ridiculous; there's nothing like this in my job description, but I can't say no for some reason. Am I afraid of getting fired, or am I just naive? I don't get it. What kind of attitude do you think I should adopt?