To process a quick sale with an account balance or cash, click the item that you are selling and then click Go To POS.
You'll be brought to the Add-Ons screen. If you'd like to add additional products to the cart, you can do so here; otherwise, you can see that the item you selected for a quick sale is already in their cart. This is indicated by the "1" next to their name. Next, select Charge and confirm the items in the cart. Click Proceed to Payment.
Click on the toggle to apply the account balance, or enter the cash amount and click Submit Payment.
If using account balance, the Submit Payment button will only activate when the customer has a credit card on file, even though the customer is paying by account balance.