Upvote:2
It seems that you do not need to purchase the shipping label from VFS unless you wish to use their premium service. You are free to use a pre-paid, self-addressed UPS shipping label for passport/document return. Quoting from VFS FAQ:
8. Document return
Customers who attend a Premium Application Centre, will have their passport returned to them by courier following the decision on their application, as part of the Bronze, Silver, Gold or Settlement Package purchased.
Customers who visit an Application Support Center for their biometric appointment are advised to use one of the UPS courier services offered by VFS Global. If you choose not to purchase this service from us, you may use a courier company of your choice to send your passport/documents to the UKVI Visa Processing Hub, but you must provide a pre-paid, self-addressed UPS shipping label for passport/document return.
This seems to be backed up by this other similar question: How to generate UK Visitor Visa Shipping label for VFS Global