How to add your own custom payments
First you will need to create your new payment.
Go to Sales>Manage Register Payments
Click on New Payment Method
Once you've fill out the two fields click on Save
Next, you have to add the new payment option to your register.
Go to the System Menu > Manage Register
Edit the register you need to modify
Simply click on the new payment option and save (Note: seven is the maximum number of payments)
Open the register you just changed and you'll be able to select the new payment method.
You'll have to change all of the registers individually for the new payment.