WooCommerce POS allows you to quick apply discounts at the point-of-sale.

Item Price

Clicking on the item price will allow you to alter the price on-the-fly. The item price number pad allows you to enter a new sale price, or a discount as a percentage.
Reducing the item price by a percentage using the number pad

Order Discount

You can apply an order discount by adding a negative Fee to the cart. Click on the title to rename the Fee. Click on the arrow to change the tax status and rate.
Using a negative Fee to apply an Order Discount