UPS will not ship to PO Box addresses. So, we need to disable the UPS if the customer address has PO Box. USPS has to be shown, as it ships to PO Box addresses. So, here is a code on how to disable the shipping method for PO Box address.
Modman is the best way to arrange your magento code with custom modules. Keep your magento codes separate from the custom modules. It will help you to upgrade magento easily and also, upgrade you custom modules also.