USCIS has announced that it will not use any pre-paid mailers (e.g. self-addressed FedEx envelopes) for two weeks after premium processing starts for H-1B cap-subject petitions.
During these two weeks, USCIS use regular mail to send out the final notices rather than any pre-paid mailers that were included with the H-1B filing. USCIS is doing this "due to resource limitations as we work to process all premium processing petitions in a timely manner."
Once the two week period ends, USCIS will resume using the pre-paid mailers included by petitioners.
Elaine Martin has been practising US and global immigration law since 1997. She is an immigrant herself (from Ireland), so has a special understanding of the legal and emotional challenges involved in relocating to a new country.