OK, I know some of the complications, having written a billing system myself, but some people just strive to be inept and not get paid.
We have people that insist on putting bills on some sort of portal and emailing us saying it is there - but it isn't (major telco).
We have people that insist on sending a pro-forma invoice. That I do not get. It makes no sense to me. I suspect they are trying to do a VAT fiddle, i.e. not pay VAT on services until someone pays them - but the VAT rules are clear that if the good/service has been provided then there is a tax point and VAT due with only a very limited window to move it to a later invoice date. So I don't understand that. Basically, we don't pay proformas for services we have already had.
The best one is an accountants bill. They should know better as well. They send a "request for payment" which is not even a proforma invoice. The terms are "payment within 30 days of invoice", but it states "an invoice will be raised when payment is made".... So payment is never late as an invoice is never raised. We have this discussion every year when they say we are late paying and we explain that it is not 30 days from invoice yet so can't be late. They eventually raise and invoice and get paid within 30 days. Why do they do this? Arrrg!
Of course some people just don't send invoices at all but get paid via a factoring company that assumes they have and then chases us. That is always fun.
Then of course there are data centre invoices. Always a challenge. One data centre send a separate invoice for each rack or couple of racks (so lots of invoices), but are totally unable to say which rack each invoice is for (!?!??!). Then they invoice for more racks than you actually have, every month. You can see why we don't pay them by DD :-)
The irony is that we are very careful to pay everyone on time and if only they would actually send a proper invoice on time then we'd pay them. Indeed, most suppliers we pay with order as we don't really like credit.