Thank you for your question. This can be achieved easily with a quick modification of a template file – the steps are provided below.

Just a quick note, make sure you will not be breaching any merchant agreements or local regulations by setting the agreement as a default setting.

1. Open the tpl_checkout_payment_default.php file located in the /includes/templates/template_default/templates/ directory.
2. Find this section of code:
   
3. and modify the default status of the checkbox (shown here on line 31):
   
4. Save the modified tpl_checkout_payment_default.php file to your override folder. This will be /includes/templates/CUSTOM/templates/ where CUSTOM represents the name of your current template folder.

Hope this helps,

Kerrin