Before enabling In-Cart Delivery Options for Shopify, make sure you meet the following requirements.
You must be on a Shopify Advanced plan or higher.
Alternatively, you can also gain access by switching your Shopify plan monthly payment to an annual payment. Or you may request access to Shopify's third-party calculated shipping rates that Shopify charges an additional fee for.
After you register for Real-Time Calculated Shipping, there may be a delay from 30 minutes to 24 hours before the ShipStation rates appear in Shopify.
While in the Edit Rates options, enable the option "Automatically show new shipping services to customers when they become available".
In the Manage Rates pop-up, select the checkbox next to Standard Delivery in the Edit Rate pop-up, press the Done button, and click Save.
Then you can continue setting up In-Cart Delivery Options within ShipStation.
If you are prompted to update your Shopify plan in ShipStation when setting up Delivery Options, but you are already on an eligible plan, this means the Carrier Services endpoint has not been enabled for your Shopify account.
You must contact Shopify and request that the Carrier Services endpoint be enabled on your Shopify account. Once enabled, you can then continue setting up Delivery Options for your Shopify store in ShipStation.