Does anyone know how to create a auto reminder to customers in NetSuite when invoices will be due soon. E.g. NetSuite will calculate the due date and send our customers reminder with copy of invoice(s) in ONE email. And make sure all due invoices are in ONE email instead of multiple emails.

have you looked into the Dunning Letters module? This will handle this out of the box

Hi Mika, yes I've checked the Dunning module but it only sends an email notifications on overdue invoices and not not prior to it's due date.

You have tested this with a minus value in the due date and it doesnt work?

Ah yes, I'll try on that, thanks Mika!