Hi, @jeffchiz !
Thanks for reaching out to us about this. If First Class rates are showing up, this tells us they're likely still set up in the stores shipping settings and we'll want to remove them. From your Shopify admin, click Settings > Shipping & Delivery. You'll then want to click Manage Rates in the correct shipping profile. From there, find the Shipping Zone (Domestic, International etc.) that has Carrier and app rates listed. Open Carrier and app rates and click the three dots "..." to the right of the listed rates and click Edit rate. From this page, you'll be able to add and remove the different service types as you please. You can toggle off all of the First Class rates here.
Let me know if you have any questions!