A good way to include snippets of code in both posts, pages and Event Manager Template files?

You can do this by adding a function like below in your functions.php file:

function my_cancellation_policy () {
    return 'Your text here';
}

Then you can print this way:

echo my_cancellation_policy ();

If you are about to put html code in there make sure you don’t mess up with the single and double quotes.