Radius CRM has the option to send an invoice to a customer which is related. Also, the PDF invoice will be attached to the email. To send it by email in Radius CRM, simply access the invoice to send from the administration area then click on the envelope icon.
A new pop-up window will appear after clicking on the envelope icon.
If you do not want to attach the PDF invoice, simply uncheck the checkbox Attach Invoice PDF.
After sending the invoice to the customer, if you want to send again the invoice, there will be another email content (the initial invoice is already sent). If the customer want the invoice again and in all cases, this content will be different. Go to Setup-> Email Templates -> Invoice Already Send to Client to edit the email content when sending the invoice to your customer for a second time.