Elias here with Shopify Support. That's a great question!
There isn't a settings to enable or disable First Class Mail for Shopify Shipping labels, as the shipping methods are based on the packages and shipment weight. It's also important to note that First Class Mail does not include tracking or insurance; however, First Class Package includes tracking.
For an order to qualify for First Class Mail it must be:
* 13 oz (0.8123 lb) or less
* In an envelope larger than 11.5 x 6.1 inches and no bigger than 15 x 12 inches, link here.
* Or in a soft package less than 0.75 inches thick.
* Boxes do not qualify for First Class Mail rates.
For an order to qualify for First Class Package it must be:
* weight: for Shopify Shipping 15.999 oz or less
* weight: when using your own account 13 oz (0.8125 lb) or less when using retail pricing, Page 7 here.
* weight: when using your own account 15.999 oz (0.99 lbs) or less when using commercial pricing, Page 17 here.
I hope this helps clarify the USPS First Class shipping method. If you have any other questions on this topic, don't hesitate to let me know!