Buy shipping FBM

Hello, in my shipped settings I offer usps first class as the only option. I have it set to where Amazon is supposed to automatically estimate delivery dates. But many times when I go to buy a shipping label through buy Shipping, it doesn’t offer first class so I am forced to buy priority mail even though I don’t offer it in the settings. Also I am shipping within the 1 day handling time I have it set as. I really don’t get it.