Europ Assistance- bad payment terms and business pratices when dealing with Service Providers.

As a service provider to Europ Assistance, I would like to state that their internal payments department is an absolute joke. We are now 35 days post invoice submission. It took me a few weeks to initially confirm that our invoice had been received by them, only for Pieter Van As to come back and tell me that there was info missing off our invoice (despite having always submitted our invoices in the exact same format) and that i needed to rectify it. I duly did, stating that i'd never been asked to do that before. After querying again as to when payment would be made, i was told that we will only be receiving our payment on the 24th of August, which will make our invoice submission to invoice payment, 53 days, whereas they state payment terms are 30 days. Now, not once did anyone from their company make me aware that there was an invoice issue, between my submission and when i finally got ahold of them (approx 3 weeks after submission), it was literally me having to email repeatedly to get feedback and only then did i find out about the invoice problem (again i state that this had never happened before with other invoices). Besides this disgusting delay in us receiving monies owing to us, because of Europ Assistances's inneficiencies, Pieter Van As (External Cost Omptimisation Administrator, QPC department) refuses to reply to my emails. I have put read receipts on and he opens them every single time, but simply refuses to reply and assist in getting our payment donaccording to their own companys' 30 day payment terms. I am absolutely disgusted with the way you brush people off who you owe money too. It is absolutely ********* and extremely bad business ethics.