Go to the customer database and use the search bar in the upper right-hand corner to find the customer you're looking to invoice.
Click on the customer information to be taken to their profile.
At the bottom of their customer profile, you'll see their customer history. Select which invoices you would like to send by checking the box at the left of the invoice information. Once the proper invoices have been chosen, click the 'send' button.
If the customer details don't contain an email address, add it when prompted. Then press the 'email' button in the bottom right. The customer will receive a single email with the invoices.
If Pay online is enabled, the customer will be able to click a link in the email and pay for all the due invoices in a single payment.