Dunning letters to customers based on the payment

We have a requirement in which we have to send dunning letters to customers based on the last payment made by them on the overdue invoices.

For example:

Our setup is something of this sort,

1. 0.-30 days overdue - Dunning soft letter 1
2. 31-60 days overdue - Dunning moderate letter 1
3. 61- 90 days overdue - Dunning hard letter 1

We have a customer A who has 10 invoices which are delinquent and

1. we use the days over due method and send a 'Soft dunning letter 1' after 30 days. Customer ignores this
2. We send another 'Dunning Moderate letter 1' after 60 days overdue. The customer responds and makes payment of 8 invoices