Help Please! Spouse Visa from USA - Do I have to use VFS?

Hi there,

I completed the online application form and booked an appointment for my Spouse Visa in the NYC Visa Center. However, it has among the "Next Steps" to go to the VFS website. I did this, but it is only giving me one option which is a $2K premium processing fee.
If I have already paid for priority processing on the UKVI website when applying for the visa, do I have to pay via VFS as well? Or is VFS just meant to be the postage service? Is it required when going to the interview to have purchased VFS service or can I just buy a prepaid envelope at UPS instead?

Thanks so much in advance for your help!