You can add the method without any extension support.
* Configure tt_products extension as specified in the extension document.
* Add the following typoscript into the setup part of your template.
radio = 1
10.title = cash on delivery
‘radio = 1’ is used for implementing radio button. if this is not specified then by default drop down will be implemented.
If there is more than one payment mechanisms are used, then the number 10 (10.title) will be changed accordingly.