It can be any delivery service she wants to use, who will deliver the documents to the UK.
I would not recommend using USPS if she can help it, because it will take longer and it's more difficult to track.
If she can afford it, she would be best to use a fast FedEx, UPS or DHL service, so that the documents arrive in Sheffield quickly and they are tracked and signed for (especially as she may need to wait a few days/couple of weeks for confirmation of receipt by UKBA... at least if she tracks the package, she will definitely know that it has arrived).
She will also need to include a pre-paid, international return waybill/shipping document and suitable packaging... the delivery courier she uses should be able to sort this out for her.