An Allmoxy team member can turn on the Admin setting and then you can go to the Financial Settings and set you cash-payers' discount rate!
Here's some information about how cash pay discounts work:
Customers do not have the Manual Payment option, because this should be either cash or check that your office staff is receiving, then applying.
Selecting Apply Cash Discount creates a credit memo in the system which is then applied to the invoice so that the customer only pays the discounted remaining balance. 
