This is a good topic. I think you should ask the FDA if you are not satisfied with the answer of the cashier. After i read your post, i think the two of you have different perceptions with the subject that's why you can't let go also to what you are trying to win. But looking deeply into your conversation, you are concerned about the plastic that in order to use fewer paper cups and plastic lids you wanted to use a reusable cup. You have your own opinion about the reusable cup. But the cashier also trying to explain what she thinks about using reusable cups which is also a reasonable for her not to allow you to use those cups. I think you need to understand also the perception of others. Think that maybe other people around you have Health-conscious than you are or others were also like you, that wanted to save the mother earth. Always remeber that "It's not what you look at that matters, it's what you see". Lets try to reflect this ourselves and maybe there will be a changes in our life.