Add USPS Shipping option for existing Magento site

We have an existing Magento ecommerce site that currently supports UPS for shipping. Visitors can purchase gift cards which ONLY have a USPS option for shipping. We want to keep that option, but add USPS (in addition to the current UPS option) for other types of orders. As it stands, visitors can get a shipping quote on their cart page by entering a zip/postal code and it gives them UPS options. We want to add USPS as well. During the checkout process, we also want to add USPS as a shipping option. I'm not terribly familiar with the backend of this site, but I imagine because of the gift card option, shipping has been customized or an extension was used. Simply turning on USPS in the Magento admin does not enable it on the site. Need a Magento expert to dive in, assess how to do this, and add USPS as described.