Putting disclosure in USPS Express mail

I wanted to put a disclosure in Express mail stating about the shipping time while they check out.

we are having complaints people see 2 day shipping and think it ships instantly and do not
give us processing time or the fact that we may be closed when they order it

How can this be done?

Thank you

If you are actually using ZenCart version 1.3.8a you need to seriously upgrade due to known vulnerabilities

You can add to the file /includes/languages/english/checkout_shipping.php where you find

define('TEXT_CHOOSE_SHIPPING_METHOD', 'Please select the preferred shipping method to use on this order.'); 

Edit what is in red maintaining the set of single quotes

Thank you. We are in the process of upgrading to the latest Zencart version