Just visit Admin > Invoices > Open Invoice Reminders to check it out;
You can create some pretty powerful reminders with this.
After N Days will just send one time per invoice per 'reminder', when the invoice is that old. You could create one of these messages for 30 days, one for 60, and one for 90 for example, with different messages. If you are setting this up when you already have people 90+ days past due, they will get duplicates - but once this is setup that shouldn't happen with invoices going forward.
Day of Month will send that same reminder message every month while the invoice is overdue (past N days). So, you could just have it send a reminder every 1st of the month to "every invoice that is more than 30 days" overdue. It doesn't allow you to change the messaging like the other type of reminder, but it does give you a different way of sending them.
- Give it a name just for fun
- Set the schedule mode, After N days or on X Day of Month
- it needs a number of days 'overdue'
- You can set a CC email so you can copy yourself to see how the system is working
- Set the from to your billing person so they get the replies
- And the email message can accept HTML formatting, but keep it pretty simple - this will be inside your fancy HTML email wrapper template, and it will have the invoice attached automatically as a PDF