This option allows you to set up a product or service invoice or a single invoice comprising products and services. To find the option, go to Settings - Accounts - Setup. 

Set up a Product Invoice

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

Set up a Service Invoice

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

Set up a Single Invoice Comprising Products and Services

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