Using Mail In Payment as a gateway will give you a text area to enter payment details for the client that will be displayed on the printable invoice. This can be used to show your address for clients to send checks or money to you. As a result the 'forward user to payment gateway' order redirect option will redirect the client to the invoice.
When you receive the payment, you would need to add the transaction manually into WHMCS by using the Add Payment section of the invoice.
This module is open-source meaning it can be easily duplicated if you wish to offer multiple options. For instructions on duplicating these modules refer to Bank Transfer.