Juvonno is able to calculate finances charges on outstanding invoices.
Go to Settings>General>Accounting>FinanceSection.
Set the following:
Monthly Interest Rate.
Grace period before charges would start to accrue.
Minimum Interest chart if you wish.
Minimum transaction value to accrue interest on.
Finally, select when you want to calculate interest charges from, Invoice Date or Due Date.
Save your settings.
Now when an invoice meets your criteria set above, the interest value will be calculated at the time the invoice is being paid and will create a new interest invoice.