Send your customer an email with the invoice and link to pay online
Navigate to the invoice that you are working on. Once you are there, click "Send" and then "Email"
Once you click "Email" the system will open up the default email client where you can customize the message to the client but it also includes the link to the invoice:
Once they click pay, the client will be navigated to the payment screen as show below which they can proceed with payment: