Hi all. I work in a post office branch and we check 70-100 applications a week. Many people value the service, and after 25 years I've lost count of the number of incomplete and incorrect forms I've seen. For those people the checking fee (now £8.75) is well worth it. The apps also sent securely, which would cost you £5.90 if you did it yourself, as well as having to enclose bank payment details on the debit form. Security and peace of mind.
For what I'd call a 'normal' first passport, with the applicant and parents born in UK the full birth cert would be enough. If anyone was born o'seas then the passport office need to see relevant passports. That's to provide evidence of claim to nationality. Sometimes they ask for further ID before issuing 1st passports which is out of our control. The clerk has a check-list to use for the ID required to be sent, and experience of checking the forms helps. Was it a main branch or a smaller one ?
I'm guessing the IPS told your son to wait indoors ? maybe they wanted to conform the address / ID ? all of my families passports have come via the IPS courier and have just been posted through the letterbox, even though I've been at home when the last 2 arrived. They don't knock.
Good news is he's got the passport for his trip, and a renewal will be much easier.