You can set up a product or service invoice or a single invoice comprising products and services. To get to the relevant page, select Settings-->Accounts-->Setup. 

Set up a Product Invoice

1. From the SALES INVOICE METHOD list, select Products Only and click Save to save the settings.

Set up a Service Invoice

1. From the SALES INVOICE METHOD list, select Service Only and click Save to save the settings.

Set up a Single Invoice Comprising Products and Services

1. From the SALES INVOICE METHOD list, select Products > Services and click Save to save the settings.