How do I add payment options for customers... IE cash and check options?
In their company settings you can add credit card and bank accounts (the same place you go to change their address or their permissions, etc). Customers can also set up their own payment methods in their company settings as well! You'll add the bank or credit card payment methods by using the "Manage Payment Methods" link in the settings:
Adding a bank account will allow them to use the "quick check" option when they make payments. You don't have to set anything up for a cash option or regular check option - in that case they'll give you the cash or check and your staff will be able to process the payment with the "Offline Payment" option. This allows you employees to choose whether they are paying with cash or check, etc.
Please let me know if you need more guidance on this process!