Short answer
If your products are fulfilled by Fourthwall’s partners, shipping options are automatically calculated and can’t be changed manually.
More details
Fourthwall determines the best shipping method and carrier for your customer based on location and the product type. For example, a mug and a hoodie ordered together may ship separately because they come from different fulfillment centers.
If you’d like to offer your own shipping options (like Media Mail or Priority Mail), you can do that, but only for self-fulfilled products. To do this, switch your product’s fulfillment type to I’m shipping orders myself (and make sure to connect ShipStation for ease of fulfillment).