Under Accepted Payments, there is an option next to Pay Online, Pay in Person called "Other Method".

Other Method can be used in multiple ways. If you don't like the "Pay in Person" and specifically want to say "Cash", it can be used to display "Cash" as 1 of the payment options which the customer will see while finishing up their order.  See below:


It can also be used to accept Gift Cards or special in house accounts, where the customer can pass along the information to you and then you can add the charge to their account or process the gift card manually. Please note: Special closed system gift cards cannot be processed online through this, just the information will be passed to you. It can also be used for special accounts you might have for some businesses/organizations.  

The Name would be "Gift Card" or "House Account"  Description - add a description of what this other method is used for "use your gift card with us" or "enter your in house account number with us"

Option 1 and Option 2 are both fields you can name and get that information from the customer. They are not required and you can use only 1. For example Option 1 could be "Gift Card Number" or "In House Account Number" This will give 1 field for the customer to enter their gift card number or the in house account number for you.