This is a matter of plugin configuration. We are not experts on USPS and we can not give you much better information than you already have.
I suppose you have checked only the First class option. If I understand correctly USPS provide both the parcels and letters as first class services. I think that is why you are getting both. Maybe you should select only the parcel options (Bound Printed Matter, Media Mail Parcel, Library Mail Parcel), but I am not exactly sure if this is what you need.
The best you can do is write in the nopCommerce forums
. Maybe a store owner who is using the USPS could give you better advice on how to configure it the way you want.
Hope this helps!