Hi,
would like to know if it would be standard USPS service to mail documents for the visa application to the UK then just the return postage through vfsglobal at $141?
I would NEVER advise using USPS to mail important visa documents under ANY circumstances. USPS are not particularly secure, the package cannot be tracked properly and there's a good chance the documents will get held up at customs, possibly for several weeks, and they may never arrive in Sheffield at all.
I would ONLY use UPS, DHL or FedEx to mail the documents to Sheffield, and then you can purchase the $141 VFS Global label for shipping back to the US.
Alternatively, you could purchase both shipping labels from VFS Global for $227, as they are just 2-day UPS shipping anyway, and it'll likely cost you a similar amount to ship it to Sheffield yourself using a courier anyway.