Oh man! As consultant I hated that. They would pay 90 days late and just say "sorry, we always pay 90 days after we get the invoice". They know I don't have the resources to sue them, nor would I. And they really had no reason to hold it other than making sure they get the interest instead of me.
Sure maybe it takes a few days to validate the invoice, but 90 days was ridiculous.
Even worse was when I was on the corporate side. I was working with a small vendor and our company had a 90 day wait as policy, but the company refused to deliver the items until they got the money. I really needed the items, so I have to move mountains to get our procurement to cut a check ASAP.