Hong Kong visa return address.

I am a Nepalese citizen currently residing in the US. I am planning to apply for a Hong Kong visa. Over the HK immigration website, it states that: "Please ensure your mail items bear sufficient postage with return address before posting".

How do I get a return address mail with sufficient postage? Do I need to get it from HK post or any USPS or UPS or FedEx return post will work?

Thanks.