Some people must also be bored enough to not completely read through a comment and comprehend what's being said before dropping their 2 cents. If you require a specific document put it on your billing instructions, pretty much every confirmation has its requirements, and if you do require something be forthcoming when you receive the invoice not when it's overdue and is being inquired about, that's scummy. I can name 20 brokers off the top of my head that don't require confirmation with their invoices. Every organization has it own billing instructions, not everything is common practice, just had one last week ask me to mail the invoice after we emailed it thinking its common practice lol.