I'd like to ask, what kind of travel invoice do I need to apply for Hong Kong Pass? Must I go with the tour group? Can't I do it myself?

Travel invoices required for handling Hong Kong and Macao passes are only documents required for handling passes. You can find an acquaintance to help you find a qualified travel company to open it. Then generally provide ID cards and household registration books to the exit-entry administration department of the Public Security Bureau for payment. As for who to go to Hong Kong and Macao with, nobody cares. As long as it is within the time limit (generally within 6 months after application), it will be invalid after expiration.

I know because that's what my relatives do. Good luck getting the certificate:-)