Upvote:2
VFS Global is not a courier service, it is the company that processes your application, meaning they carry out "administrative and non-discretionary tasks" related to your application.
You can mail your documents to them by any means you like.
The "fully addressed prepaid electronic shipping label" that you are required to send for your visa to be returned can also be from any company.
Here is the web page: https://www.gov.uk/government/publications/usa-apply-for-a-uk-visa/apply-for-a-uk-visa-in-the-usa