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Send Individual Attachments - each invoice invoices will be send in separate sent as individual PDF files within one email
Send ZIP Attachment - all invoices will be in a sent within one zipped file
Note:
The system sends only posted invoices that have a status To Send - to send status have only those invoices, which are posted. In case you leave field Send Rental Documents empty, the system will not send invoices to this customer.
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choose the status of the invoice in field Rental Send Status
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Follow up:
How to create reminder in general