I was able to find the answer myself.  Basically use customer payment terms.  Note that the invoice itself is not changed with finance charge since, but a charge is shown on customer account for example when you take payment.  (Have not figured out how you inform the customer about the finance charges yet.)

for examle setup payment terms: "30 days, 1% finance charge per month late"

Then add computation items:

Computation = Percent, Number of days 30, Amount to pay 0

Computation = Percent, Number of days 60, Amount to pay 0.01

Computation = Percent, Number of days 90, Amount to pay 0.02

you can add additional, but if customer has not paid in 90 days you have a bigger problem.

Info was gleaned from this question:  https://www.odoo.com/forum/help-1/question/is-there-a-possibility-to-rise-dunning-fees-or-give-early-payment-discount-v7-and-v8-36327

Hi Mark,  please find this module in OCA  https://www.odoo.com/apps/modules/10.0/skit_dunning/ By this you can send them remainder email.