As for using their courier, we had a significantly larger amount of documents due to category F and a long marriage, so their service worked out cheaper in our case
For the record, VFS isn't a courier service, they are just UKVI's commercial partner through whom you can pay for other people's services. They run a website and that's it.
- The settlement priority processing service is provided by UKVI in Sheffield
- The shipping 'service' is just pre-paid UPS shipping.
It may just be easier and cheaper to do the shipping through UPS directly and cut out the middle-man (VFS).
Another interesting (worrying) thing was that the email acknowledging receipt of the application came only 10 minutes before the email asking for courier information. Hopefully this doesn't indicate a swift rejection - I was expecting at least 2-3 days between the initial email and a decision email or an email asking for courier information.
No, they most likely just check that you have included a return shipping label before they start processing the application. Someone probably just opened it, went to check that you had included the shipping label and then emailed you about it... so that they have everything in order before they hand it to anyone for processing.