SAB poor financial system

We received a letter of demand from SAB, since last year October we have been requesting signed documentation, since we cannot buy from SAB as our orders are to small.. We receive many phone calls for payment and emails, but they never respond to our request for the documentation leading to the outstanding amount payable as per their records. Even mailed the Financial manager on 7 February 2019 and still we are waiting for the invoice and pod. How do you expect us to sort out the account if you cannot respond to our requests?