SimpleInvoices.io – great for sending electronic invoices and simple recurring payment setup. This is an easy front end to Stripe.com. You will need to first setup a stripe account.
Useful 3rd Party Tools
Appointlet.com – Book appoints on your website with calendar syncing to google/microsoft/other calendars. Nice FREE tier available.
Cal.com – Book appoints on your website with calendar syncing to google/microsoft/other calendars. Nice FREE tier available.