up_subtotal provides the following buttons/commands:
STCA - Set the order's payment method to 'POS Cash'
STCC - Set the order's payment method to 'POS Credit'
STCK - Set the order's payment method to 'POS Check'
The store administrator can set up conditional actions to alter the order based on the payment method. For example, the uc_fee module can be used to charge a fee depending on if the customer is payment with cash or with a credit card.