The PH Multi Carrier shipping label app supports USPS First Class Mail. As you rightly mentioned, USPS First Class Mail is applicable for orders that weigh 1 ounce at most.
Generally, once you set up your product weight, envelope size, and shipping rate, the correct rates should be displayed to your customers. However, if a customer has more than one envelope in their cart, a different rate may be applied due to the weight and envelope requirements. This could explain why the USPS Priority Mail rate was displayed instead. You can learn more about packaging here.
We will need to access your account to gather more information. However, please note that we are unable to provide account-specific support through the Community Forums. But don’t worry! We would be more than happy to continue assisting you through our Help Center. To access our live support, you can ask the Virtual Shopify Assistant to “contact support.” It will guide you on the next steps to connect with a team member who can provide further assistance tailored to your account.