On the ribbon, go to Items and create a new Item called POS for template POS Jobs, mainly so its easy to distinguish between retail sales and other types of Jobs. You can also hide the Comment grid if you wish.
In this Item, you can also add stock, if desired. For instance, you may wish to have the user ask a customer if they would like a particular item (ie. upselling, etc.). You can create a stock code and add this to the Item, which will then appear in the job.
The same applies to using Watchouts. You could add a Watchout to tell the user to try and upsell to the customer.
Further information
Invoice Authentication Security
Settings to Speed up Invoicing