Connect a second monitor facing the counter, then select it under Settings → Hardware → Customer Display. It shows addresses, shipping rates, sale items, and totals during a transaction. When no transaction is in progress, it shows your logo and brand colors. Set those in Settings → Branding → Customer Screen.
To run a slideshow between transactions, put your slides as image files in a folder and choose that folder in the Customer Screen settings. If customers have trouble reading the display, increase the font size there. If the customer display is a touchscreen, customers can also sign customs forms and mailbox agreements on it.
If you pick the wrong monitor and PostalPoint takes over your main screen, click anywhere in that window and press Alt-F4 to close it, then choose the other monitor.