UKF - Return of supporting documents

[aakashbasu2]

Hello, I am applying to register myself as a British citizen via form UKF. I am applying from the US. The application guide says that I need to send a pre-paid self-addressed Royal Mail envelope, should I want my supporting documents returned in a secure fashion. I don't know how I can get a royal mail envelope in the US, and moreover, I would really prefer that the documents be sent back via a courier service like FedEx or DHL.

Could anyone advise me on whether I can send them a pre-paid DHL envelope instead? As I am in the US on a visa, I really really don't want my passport lost in the process!

Could people who applied using form UKF from the US share their experiences on the return of their original documents?