Shipping a UK Visa Application in the USA

You raise two questions.

The uk.gov page uses the term "documents" to refer to the whole packet, i.e., the application form, your passport, and supporting documents. I can understand only wanting the passport back, but were I you I would want to avoid a semantic dispute with VFS about whether the word "documents" includes your passport or not. For the greatest liklihood of getting your passport back, enclose a prepaid electronic shipping label, as the uk.gov webpage requires.

You also ask if you might ship your application by UPS. I have no personal experience with this issue, but I see that Section 3.1 on the uk.gov page linked above itself links to VFS. This is just like the first question: the web page tells you what to do — even linking to VFS, who, presumably (when you log in) tell you what kind of carrier to use. For shipping your application packet to VFS, use the carrier recommended or required by VFS, or a carrier that will employ a "fully addressed prepaid electronic shipping label" as required by the uk.gov webpage.

Taking either of the courses of action you propose (mailing without enclosing a prepaid electronic shipping label, and sending by UPS) could imperil VFS's processing of your application. You might ask yourself why you don't just do what they're asking you to do. I think you'll be much better off to follow the instructions.

EDIT: You might also review this SE:Travel Question and Answer, which addresses the electronic-ness of the label as required. If the UPS shipping label has a barcode, I think it'd qualify.

I my experience with UK visas, although not in the USA, they arrange shipping for your passport, and send your supporting documents separately - using whatever you sent in. So yes, if you don't send a shipping label you don't get back your supporting documents, but will still get your passport.