why did shopify automatically create a label for my order

Hi,

I’m a first time user with shopify, so I am having some trouble understanding the system. When creating shipping labels for my orders, they automatically are creating labels for a medium flat rate usps boxes when some of my orders don’t require that box size. I tried editing it on my saved packages page but for some reason there isn’t an edit option next to it like the rest? How do I go on about this?