As Merchello ships with the cash provider enabled by default. It's a very simple payment option with no configurable options.
Merchello does have other Payment Gateways, which can be found in the Github Repo and this is also a great place to start when you are looking to create your own Custom Payment Gateway.
Payment gateways (And other gateways) are installed just like Standard Umbraco Packages.
Updated less than a minute ago