Invoices - allow to sent multiple, independent ones

In the world of invoices public school systems and universities, we need an invoicing system that works like an invoice and not like an account statement. When an organization is billed for their tournament/court usage we get a PO #. We need a place on invoices to note that. Then it typically takes 30-45 days for organizations to process payment. In the meantime they might have more events/fees and need an invoice for those. However, the current system will loop in the old fees that are already in payment process. I cannot sent a new invoice with old fees on it. I need a new invoice with the current fees only -regardless of the payment status of previous invoices. We have only started using CR invoicing this year and it's become a major issue for our facility. I can't imagine there are not other facilities that invoice companies that need professional invoices to send. This includes an area for notes and PO#s and also the ability to pick and choose what the invoice is for that is independent of any past invoices sent. Thank you!