How do I email an invoice to a customer automatically?

If you have a customer who prefers to receive invoices via email, you can set that customer's preference in Street Invoice, and then, when you create a new invoice for the customer, the email sending option will already be set.
(You can still choose not to send the invoice on an individual basis.)
Here's how to set the customer preference:

Mobile

From the Main Screen, select Customers.
Select a Customer.
Tap the pencil icon to edit the customer.
Tap the Send Options tab.
Check the types of documents you would like to automatically email to the customer.
Under Send Via, select Email, and either choose an email address from the dropdown menu, or enter another email address.
Tap the Checkmark to save the change.

Now, when you save a new invoice for the customer, the invoice will automatically be sent by email.

Windows PC

From the Main Screen, select Customers.

Select a Customer and click Edit Customer.
Click Sending Options on the right.
Check the types of documents you would like to automatically email to the customer.
Under Send Via, select Email, and either choose an email address from the dropdown menu, or enter another email address.
Close the window to to save the change.