USPS returns CLASSID values for different services, but in a terrible design, they have a handful of CLASSIDs that map to different services, with different rates returned. If you do a rate request for a package under 1 Lb you'll see this in the response.

Here's an example in the response for an 8 oz package:

		<Postage CLASSID="0">
			<MailService>First-Class Package Service - Retail&amp;lt;sup&amp;gt;&amp;#8482;&amp;lt;/sup&amp;gt;</MailService>
			<Rate>5.40</Rate>
		</Postage>
		<Postage CLASSID="0">
			<MailService>First-Class Mail&amp;lt;sup&amp;gt;&amp;#174;&amp;lt;/sup&amp;gt; Large Envelope</MailService>
			<Rate>2.88</Rate>
		</Postage>

Karrio just maps the CLASSID to a service type, so both of these end up getting mapped to usps_first_class, but that's confusing as in this case one would only qualify if shipping in an envelope.

Since it is very unlikely you are going to be able to convince USPS to fix their one-to-one mapping of CLASSIDs, I think you'll need to parse the MailService strings to differentiate between these.