Hi Howard,
If you want payments to automatically apply to the oldest outstanding documents (be they invoices, credit notes, debit notes, interest, etc), I believe the way to do this is to set the Customer to be 'Balance Forward' rather than 'Open Item'. Thus when you enter payments, you simply enter the payment amount and when this is posted, Adagio Recievables automatically calculates which invoices are paid, applicable discounts, etc. You can also select specific invoices to apply to.
This is set by Customer, so you can have some customers work like they still do, and other customers use the method above.
Before you start changing the Customer Type to Balance Forward for your customers, you should also be aware that other functions are affected by this change. For example, your statements will show different information for transactions posted prior to the last Month End. You should try this out on a copy of your data before attempting to go live with this.