Adding shipping existing shopping cart
I have an existing shopping cart (ecart6) with custom shipping formulas. I want to delete those formulas and use a link to the USPS instead. When I go to edit the cart, I can delete the formulas, but there is no longer an option to add the USPS. I tried clicking on behaviors (I saw that tip in another post), but there was no shipping behavior to modify or add. Do I have to totally recreate the cart? I don't know enough about coding in this language to do it manually.